The change in the microphase separation transition temperature (T-mst) of a diblock copolymer (AB), induced by the addition of homopolymer (HA), is theoretically studied. The calculations based on the modified Meier’s theory show that T-mst is dependent on both the volume fraction of added homopolymer and the molecular volume ratio of HA to AB, which is the same as the previous theoretical results and qualitatively in agreement with the experimental results. This theory also predicts that the molecular weight ratio at the crossover of T-mst from elevation to depression is affected by the molecular weight of AB.