A corner trimmer includes a base, a bottom blade, an L-shaped operation member, a top blade and a spring. The base has a platform and a pivotal portion. The bottom blade having a round edge is fixed to the base and parallel to the platform. The two ends of the L-shaped operation member and two ends of the pivot of the pivotal portion are connected to the base. The L-shaped operation member has an opening. The top blade is located at the inside of the L-shaped operation member and has a curved edge which is located corresponding to the round edge of the bottom blade and shaped to be complementary to the round edge. The spring is biased between the base and the top blade. The corner of an object is inserted into the opening and trimmed by the alternative top and bottom blades.