Casts

Humans

• Shia LaBeouf as Sam Witwicky: An offbeat eighteen-year old who buys his first car, which turns out to be Bumblebee. Sam ultimately becomes the saviour of humanity.
• Megan Fox as Mikaela Banes: Sam's girlfriend. Her father was a "grease monkey" and she has inherited some of his skills.
• Josh Duhamel as William Lennox: Captain of a Special Ops team in Qatar, who manages to escape the destruction caused by Blackout. He has a wife and child back home.
• Tyrese Gibson as Staff Sgt. Epps: An Air Force Combat Controller who teams up with Lennox to escape Blackout and Scorponok.
• John Turturro as Simmons: Head of Sector 7, a secret government group that holds Megatron hostage. Simmons is leading a team tracking down Bumblebee.
• Jon Voight as John Keller: The United States Secretary of State.
• Kevin Dunn as Ron Witwicky: Sam's father.
• Julie White as Mrs. Witwicky
• Rachael Taylor as Maggie Marconi
• Anthony Anderson as Glenn: A computer hacker who is hired to decipher the Transformers' language.
• Bernie Mac as Bobby Bolivia: A car dealer from whom Sam buys Bumblebee, both unaware of its alien origin.

Autobots

• Peter Cullen as the voice of Optimus Prime (Semi-trailer truck): The powerful, wise leader of the Autobots, Prime comes to protect humanity from the evil Decepticons.
• Mark Ryan as the voice of Bumblebee (1974/2008 Chevrolet Camaro): Bumblebee is an honest and friendly soldier who befriends humanity, namely Sam, in his efforts to find the Allspark. He has difficulty speaking after he was attacked by Megatron and communicates with a radio.
• Darius McCrary as the voice of Jazz (Pontiac Solstice): The smallest and feistiest of the Autobots, Jazz is one of Optimus' best warriors, talented at learning new cultures.
• Jess Harnell as the voice of Ironhide (GMC Topkick pick-up truck): A tough, weathered soldier, he is always the first in battle. Ironhide is one of Optimus' oldest friends, yet he has a darker outlook on life.
• Robert Foxworth as the voice of Ratchet (Search & Rescue Hummer H2): The medic and scientist, Ratchet has a strong sense of duty to protect life.

Decepticons

• Hugo Weaving as the voice of Megatron (Cybertronian jet): The missing Decepticon leader held hostage by Sector 7, Megatron desires power over the Allspark and hates organic lifeforms.
• Charlie Adler as the voice of Starscream (F-22 Raptor): Megatron's second-in-command, Starscream actually desires to overthrow Megatron and take the Allspark for himself.
• Jess Harnell as the voice of Barricade (Saleen-modified Ford Mustang police car) : The Decepticon hunter who masquerades in society as a symbol of the law.
• Reno Wilson as the voice of Frenzy (2-speaker CD player): The smallest of the Decepticons, Frenzy attaches to Barricade. His main purpose is spying on humans, but he is still a ferocious fighter, shooting blades disguised as CDs from his chest.
• Jimmie Wood as the voice of Bonecrusher (Buffalo H Mine-Protected vehicle): A Decepticon warrior who hates all others. He is only subservient to Megatron, whom he fears.
• Blackout (MH-53 Pave Low): The largest Decepticon, Blackout is fiercely loyal to Megatron and can fire electromagnetic pulses, making Decepticon attacks easier.
• Scorponok (mechanical scorpion): He has a symbiotic relationship with Blackout, hiding within him until he is commanded to go and kill.
• Brawl (Modified M1 Abrams): A moody warrior, armed to the teeth.

In March 2007, Hugo Weaving was revealed to be playing Megatron. TV series voice actor Frank Welker auditioned, but his voice didn't fit Megatron's new look, though he is voicing the character in the video game. Weaving's voice was used for facial animation tests.

Source: http://en.wikipedia.org/wiki/Transformers_(film)