The present invention provides a semiconductor structure, including a base, a patterned oxide semiconductor (OS) layer, two source/drain regions, a protective layer, a gate layer and a gate dielectric layer. The patterned OS layer is disposed on the base. Two source/drain regions are disposed on the patterned OS layer and are separated by a recess. Each source/drain region includes an inner sidewall facing the recess and an outer sidewall opposite to the inner sidewall. The protective layer is disposed on a sidewall of the patterned OS layer but is not on the inner sidewall of the source/drain region. The gate layer is disposed on the patterned OS layer, and the gate dielectric layer is disposed between the gate layer and the patterned OS layer. The present invention further provides a method of forming the same.