The morphology of bulk heterojunction (BHJ) photovoltaics is emerging as the center issue that determines the photonphysical properties and device performance. In this lecture, we discussed the detailed morphology characterization and structure-property relationship in low band gap polymer photovoltaics. Several important polymers are selected as the model systems in the study, in which the processing conditions and chemical structure modifications are investigated, and their influence on morphology, and thus the final device performances, are compared and discussed. Physically, parameters such as crystallinity, structure order, phase separation and vertical segregation, miscibility, and interactions of solvent components are of critical importance and determine the morphology. Synergic optimization including both materials design and device engineering is required to achieve higher performance devices