A DART board committee voted unanimously Tuesday for the fleet purchase, with service expected to begin in 2013.
DART will build CNG fueling stations, with the federal government helping with the cost. Current buses run on diesel fuel.
DART chairman William Velasco cited environmental reasons for using CNG-powered buses, saying "cleaner air is better air."
DART provides transportation for Dallas and a dozen surrounding cities in a 700-square-mile service area. The operations include the state's largest municipal rail system.
