Abstract: To cont rol the degree of fiber compaction , insure the quality of composites and reduce the cost , it is necessary to study how the state of fiber compaction changes and the factors that affect it . A parameter which is calledcompaction index I_c is used to show the degree of fiber compaction of unidirectional composite laminates. The greater the I_c , the greater the degree of fiber compaction. For the hot pressing process , the effect s of pressure , timingof the applying pressure and the number of layers on I_c were analyzed by means of optical microscope along thethickness of the laminates. The result s show that I_c increases nonlinearly with pressure , decreases with the increaseof resin viscosity at the timing of applying pressure and reduces linearly with the number of layers. These results provide some important experimental evidence to optimize the hot pressing process and establish a short2distance flow model.