"It's very important that we expand our use of clean energy and make a long-term commitment to it," Roberts said in a statement. "Biodiesel and ethanol are better for the environment and for the air we breathe."
Earth Biofuels said the Oscar-winning actress would help the company on a campaign to get school buses to use biodiesel nationwide, e!online reported.
Roberts recently appeared on the cover of Vanity Fair's first Green issue, which profiled stars' efforts to raise awareness of environmental issues. She, actor George Clooney, former Vice President Al Gore and environmental activist Robert F. Kennedy Jr. were pictured on the magazine's cover.