This paper describes isothermal and nonisothermal crystallization kinetics of a Ziegler-Natta catalyzed high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins. Standard techniques such as differential scanning calorimetry (DSC) and light depolarization microscopy (LDM) techniques were used to measure isothermal kinetics at low supercoolings. DSC was also used to measure nonisothermal crystallization kinetics at low cooling rates. Extrapolation of isothermal crystallization half-times of Z-N catalyzed LLDPE resin using the isothermal half-time analysis led to erroneous predictions, possibly due to Z-N LLDPE consisting of a mixture of molecules having different amounts of short chain branching (comonomer). However, predicted reciprocal half-times at high supercoolings, using isothermal half-time analysis and using nonlinear regression of nonisothermal crystallization kinetics measured at low cooling rates using the differential Nakamura model, of the HDPE were similar to measured reciprocal half times at high supercoolings of a similar HDPE by Patki and Phillips. It is also shown that the differential Nakamura model can be effectively used to model nonisothermal crystallization kinetics of HDPE resins. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012