Based on the Ludwik-Davidenkov-Orowan theory, a new criterion of BDT for polymer blends is proposed. In this approach fracture stress (σb) and shear yielding stress (σy) governing the brittle-ductile transition (BDT) of polymer blends is combined into a dimensionless group as Da = σb 2/σy 2. The theory dictates that BDT occurs at a critical condition Dac = 1, brittle fracture occurs when Da 1. It can be shown that Da = Fg · Lym/LD, where Lym measures the length of shear band of the matrix, LD is a parameter determined by the morphology and interfacial adhesion of dispersed phase, and Fg is related to specimen geometry and other extrinsic parameters. It is suggested that the onset of BDT depends on the competition between the Lym and LD. With due consideration to blend morphology, interfacial adhesion and matrix properties, existing experimental evidence supports the applicability of the proposed criterion.