Simple repetitive sequences are widely present in genomes of most organisms. They are closely related to molecular evolution, genetic diversity, molecular marker and some hereditary diseases. In this article, all of the 18 bp trinucleotide repetitive double strand sequences were used, and their expansion by DNA polymerase was researched systematically. We discussed the factors including reaction temperature and sequence which influence the amplification efficiency and the length of product. The result showed that almost all of the repeats can be elongated, and the variance in sequence had great effect on the amplification efficiency. The repeats with more GC, especially those with G and C in one of the strands were much easier to grow. Duplexes with one strand containing only A and C were amplified a lot as well. The optimum reaction temperature and the GC content of the duplexes had a positive correlation. Agarose and polyacrylamide gel electrophoresis showed that the product has narrow size distribution of molecules, and the expansion has a linear dependence of the length of products on the reaction time. Differentiation of the products occurred more at higher reaction temperatures. At last, a slippage model demonstrating the mechanism of duplex expansion was discussed, and it is promising to be used for explanation of molecular evolution and unusual expansion of repetitive sequences during gene amplification and detection.