Abstract: This paper presents a fast algorithm to detect the Least Significant Bit (LSB) steganography. The new approach is inspired by the work of Dumitrescu et al(2003) who detected LSB steganography via sample pair analysis. The new algorithm combines with the statistical measures developed by Dumitrescu and a new least square estimation. A simple cubic equation is deduced to describe the length of hidden messages with relatively high robust and precision. Moreover, compared with Dumitrescus algorithm, the novel method has much lower false alarm rate of 5% than that of 13.79%, and the estimating precision is about 9% higher if the embedding ratio is less than 10%, and the speed is also about 15 % faster. Some theoretical derivations are also included.