Abstract: Matrix multiplication is widely used in scientific computing. The performance of typical matrix multiplication algorithms on Dawning 1000 are compared and analyzed in this paper. The analysis shows that matrix block size is an important factor in the performance of the SUMMA algorithm, but how to choose the best block size is not considered in the original algorithm. A criterion is proposed here for choosing the best block size in the SUMMA algorithm after theoretic and experimental analysis. Higher performance is achieved by improving the SUMMA algorithm using this criterion. Experimental results show that it can reach 50.7% of the peak performance.