The brittle-ductile transition temperatures (BDTT) of polyethylenes with different degrees of crystallinity, such as LDPE, LLDPE, MDPE, and HDPE, were evaluated using Charpy impact test. The effect of the specimen cooling condition on the BDTT was a main concern in the study. A rise in the cooling plate temperature (Tp) for high density resins brought about a fall in BDTT and an increase in the Charpy impact value Et. However, embrittlement was observed during a tensile test for HDPE samples with a rise in Tp. A crack shielding effect by microvoid formation is proposed as the toughening mechanism for HDPE.