The crack behavior of poly(trimethylene terephthalate) spherulites was studied mainly by polarizing optical microscopy, along with scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition to the effects of temperature and constraining substrate, another important factor, film thickness, on the formation of crack was first put forward and investigated. The emergence of the first crack occurred at 120 degrees C during cooling after crystallization at 190 degrees C for the sample with a thickness of 31.0 mu m. For the spherulites growing between glass sheets, it was interesting that the sample with a thickness of 26.0 mu m exhibited the largest number of crack measured per 200 mu m of radius, whereas samples thicker than 100 mu m or thinner than 1 mu m did not induce the formation of crack. Also, spherulites growing between two polyimide and two Teflon sheets showed no crack. Glass sheets lubricated with silicon oil restrained the number of cracks but did not eliminate cracks. SEM revealed that the cracking was about 900 nm in width. In addition, the AFM results suggest that the cracks had a depth of at least 150 nm. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1713-1719, 2009