Abstract. Atallah et al. [ACD01] introduced a randomized algorithm for string matching with mismatches, which utilized fast Fourier transformation (FFT) to compute convolution. It estimates the score vector of matches between text string and a pattern string, i.e. the vector obtained when the pattern is slid along the text, and the number of matches is counted for each position. In this paper, we simplify the algorithm and give an exact analysis of the variance of the estimator.