The heat distortion temperature (British Standards Method 102C) and DTA nominal melting point of high density polyethylene have been raised to 140°C by nonrandom crosslinking in which the crosslinks are superimposed on the existing semicrystalline structure. The crosslinking agent was a mixture of dicumyl peroxide and allyl methacrylate, and the crosslinking temperature was 120°C, just below the crystal melting range. In contrast, random crosslinking by the same system above the melting range at 160°C lowered the HDT (heat distortion temperature). Conventional peroxide crosslinking also lowered the HDT. The gel swelling of randomly crosslinked polyethylene was higher than that of nonrandomly crosslinked polyethylene having the same gel content. This dependence of the gel swelling on the crosslink distribution can be correlated with the HDT. The effect of γ-radiation and carbon black loading on the HDT has also been studied.