A transmission for a vehicle may include a first input shaft continuously receiving torque from a power source and having a first input transfer gear thereon, a second input shaft selectively receiving torque from the power source through a clutch and having a second input transfer gear thereon, a coupling member configured to allow or restrict rotation of the first input transfer gear relative to the first input shaft, using a difference in rotational speed between the first input shaft and the first input transfer gear, a first countershaft and a second countershaft each having an output transfer gear to engage with the first input transfer gear and the second input transfer gear, and coupling devices selectively coupling the output transfer gears to corresponding countershafts and allowing torque from the power source to be transmitted to a desired shift gear.