This paper presents an implementation of a fast twodimensional inverse Discrete Cosine Transform (IDCT) with multimedia instructions for a software MPEG2 decoder. IDCT algorithms for sparse blocks which eliminate the calculation for zero coefficients are realized by using multimedia instructions. To reduce the cycle count for IDCT, an adaptive control method for these IDCT algorithms, based on the bit rate and picture type, is proposed and its performance is described. In the implementation of a software MPEG2 decoder, the execution time for IDCT is reduced to 10% by using MMX instructions from original C program. Moreover, using proposed adaptive control, it can further be reduced to 76%.