A transmission for vehicles may include first and second input shafts to selectively receive power of a drive source via first and second clutches, first and second output shafts parallel to the first and second input shafts, a shift device including gear pairs tooth-engaged with the first and second input shafts and the first and second output shafts and having different gear ratios, one gear pair conforming to a running speed being selected by a synchronizer to shift and output power of the drive source, and a reverse idler gear provided at a reverse idler shaft to be tooth-engaged with an input gear of the first or second input shaft and tooth-engaged with an output gear of the first or second output shaft, the reverse idler gear being selected by a reverse synchronizer to transmit power from the input gear to the output gear.