The present invention provides a small interfering RNA (siRNA) that is capable of inhibiting intracellular expression of Wnt1 through complementary binding to a Wnt1 transcript (mRNA transcript) base sequence and a pharmaceutical composition for treating cancer comprising the same. siRNA of the present invention which is complementary to a base sequence of a Wnt1 transcript (mRNA) provides apoptotic cancer cell death due to inhibition of expression of Wnt1 commonly expressed in cancer cells, by RNA-mediated interference (RNAi). Therefore, the composition of the present invention comprising the same can be used as an excellent anticancer drug.