In stretch blow molding (SBM) process, the preform growth during the stretching and blowing is critical to the thickness distribution and properties of the final bottle. Whereas the thickness distribution is one of the most important criteria in the production of bottles. So this work focused on the polyethylene terephthalate (PET) preform growth using a transparent mold, through which the instantaneous images of the preform in the stretching and blowing stage were captured. By changing the delay time of the preblow, the three preform growth types, referred to as dolphin, sandpile, and two-bubble, were observed. The longitudinal and hoop stresses acting on the preform segment during the stretching and blowing were analyzed. Two parameters, on which the longitudinal and I stresses depend, respectively, were defined. Then coning the geometry and sizes of the preform, the stresses temperature distribution on it, and the stress-strain cure of the PET material used, the cause for different preform growth types was systematically analyzed. On the basis preform growth types, the thickness distributions of bottles obtained under different delay times of the preblow were explained. (c) 2006 Wiley Periodicals, Inc.