Research on the behavior of a single polymer chain during the crystallization process and the inter-chain interactions in polymer single crystals may shed light on the mechanism of polymer crystallization,eventually making it possible to direct polymer crystal engineering[1,2].Because of the limitations of detection methods,such investigations have been impossible in the past.The appearance of AFM-based single molecule force spectroscopy(SMFS)makes it possible to manipulate individual molecule and measure the inter-or intra-molecular interactions quantitatively[3-7].However,due to the complexity of the system,it is very challenging to study molecular interactions in condensed polymer system at single molecule level[8-10].Here I will report our recent work on the SMFS study of the nanomechanical properties of a polyethylene oxide single crystal by extracting a single polymer chain out of(or putting it back to)the crystal[9,10].