Effects of layer quantities and stacking sequences on L-shape composite manufacturing qualities in using OOA(out-of-autoclave) prepregs were studied. The mechanisms of air evacuated in 5 kinds of lay-ups were revealed by image analysis of cut surfaces and thickness measurements. Results show that air in OOA prepregs is evacuated in two ways. Most of the air is forced out of layers directly by vacuum before air accesses in prepregs closed. Very little entrapped air moves perpendicularly to outer layers under hydrostatic resin pressure. When a laminate contains less than 16 layers, voids can hardly be found in layers. When a laminate contains more than 16 layers, voids cannot be expelled completely during the window of vertical movement. As for stacking sequences, the synergetic effect of slip function and nest function determines the thickness and voids content of laminates. Results show that the average of single layer thickness of unidirectional layers is the lowest, and the average of single layer thickness of quasi-isotropic layers is the highest. The voids content of quasi isotropic is the highest, which is consistent with the theoretical analysis.