Simple sequence repeats(SSR), whose biological significance causes people's increasing attention, are widely distributed in genomes of many organisms. Many of them can be elongated easily and abnormal extension can directly result in certain hereditary diseases in some cases. In this research, sixty kinds of tetranucleotide repetitive sequences (TRS) and six kinds of dinucleotide repetitive sequences (DRS) of 20 nt single strands were used for isothermal amplification by thermophilic DNA polymerase. The electrophoresis results demonstrated that most of single-strand repeats, even the sequences with no complementary bases inside like AGGA, can be elongated. The results of quantitative analysis demonstrated: palindromic sequences were amplified most easily; DRS could be amplified at a broader range of temperature than TRS; DNA with more G and C, were more suitable for amplification under higher temperature; Most strands whose repetitive unit contains two same pyrimidines were amplified more easily than their complimentary ones; the concentration of products exhibited linear relationship with time. The results of restriction endonuclease digestion indicated that the products had the same repetitive unit with their original repetitive sequences. Finally, an two-stage amplification model, including amplification by intra-chain slide and mediated by hairpin-contained structure, was proposed to provide information for the study of nonspecific amplification of repetitive sequences and pathogenetic mechanisms of relevant diseases.