The study of cis-trans conformation transition of polyphenylacetylene with different substituents is very important in polymer science.Using traditional ensemble methods,a series of investigations have been performed on the stability(as well as conformational transition)of cis-polyphenylacetylene under different external stimuli such as heat,solvents,pH and electric fields[1,2].In this study,at single molecule level,we investigated the cis-trans conformational transition of polyphenylacetylene and its derivatives induced by external force using atomic force microscopy-based single-molecule force spectroscopy(AFM-based SMFS)[3-5](Figure 1).Our results demonstrate that the larger the side group of polyphenylacetylene the more stable of the cis-conformation(helix).And the polarity of solvents has an effect on the helical sense of the main-chain of polyphenylacetylene.Furthermore,our data indicates that polyphenylacetylene can go back to its original conformation spontaneously when external force is removed,which suggests that the cis-trans conformational change is reversible.