Abstract: A thinner composition used in a washing process for manufacturing semiconductor devices includes at least ethyl lactate (EL) and ethyl-3-ethoxy propionate (EEP), and preferably, additionally includes Gamma-butyrolactone. The thinner composition has high volatility and low viscosity as well as a sufficient solubility rate for rinsing photoresist on the wafer when spraying the thinner through nozzles. Photoresist at the edge or backside of a wafer can be effectively removed at a sufficiently rapid rate, so that the yield of the semiconductor devices is enhanced. In addition, any remaining photoresist attached to the surface can be completely removed to enable the reuse of the wafer, with resulting economic benefits.