This paper investigates the structural changes of polyvinyl chloride (PVC) in meltblends of a low-density polyethylene (LDPE) and polyvinyl chloride (PVC), and the effects of LDPE content and number of extrusion passes. These effects were examined in terms of changes in weight average molecular weight and number average molecular weight, polyene and carbonyl indices, color changes of the blend, and the variations in glass transition and decomposition temperatures. It was found that loading LDPE into PVC led to the formation of short-chain LDPE grafted PVC (s-LDPE-g-PVC) copolymers, via a macro-radical cross-recombination reaction, which had greater weight average molecular weight with unchanged number average molecular weight, increased decomposition temperature, lower glass transition temperature, as compared to the pure PVC sample. The dehydrochlorination reaction of PVC was suppressed by the macro-radical cross-recombination reaction with addition of LDPE, the effect being more pronounced at 13.0 wt% LDPE. For a given LDPE content, the macro-radical cross-recombination and dehydrochlorination reactions competed with one another, thus causing the increases in molecular weight average and molecular weight number up to the 4th extrusion pass. At the 5th extrusion pass, the dehydrochlorination reaction was predominant owing to a depletion of LDPE content to be grafted onto PVC molecular chains. The glass transition and decomposition temperature decreased with increasing number of extrusion passes.