A transmission may include a first input shaft and a second input shaft receiving power from an engine continuously or through a clutch; a sub-shifting mechanism and a main shifting mechanism including a plurality of sub-shifting gear pairs and main shifting gear pairs, which have different gear ratios and are respectively engaged with two shafts of the first and second input shafts, first and second countershafts, and first and second output shafts to select the sub-shifting gear pairs and the main shifting gear pairs, a one-way clutch that transmits power from the engine only from the first countershaft to the first output shaft; a second one-way clutch that transmits power from the engine only from the second countershaft to the second output shaft; and a power transmission gear set engaged between the first output shaft and the second output shaft.