This invention provides compounds, compositions and methods for modulating the expression of human p21 using RNA interference. The RNA interference molecules can be used in methods for preventing or treating diseases such as malignant tumor. A nucleic acid molecule can have a) a polynucleotide sense strand and a polynucleotide antisense strand; b) each strand of the molecule being from 15 to 30 nucleotides in length; c) a contiguous region of from 15 to 30 nucleotides of the antisense strand being complementary to a sequence of an mRNA encoding p21; and d) at least a portion of the sense strand can be complementary to at least a portion of the antisense strand, and the molecule has a duplex region of from 15 to 30 nucleotides in length.