Eliminating common defects such as voids, bubbles and poor adhesion at interfaces will increase the quality of laminated sandwich composite structure. We are experimenting and analyzing the effect of mechanical vibrations applied to the curing system of composite materials production, particularly on minimizing void content. The range of frequency of vibrations covered was from 2Hz to 8kHz, for different period of vibrations. The composite laminates were made by hand lay-up using glass fibres and vinyl-ester resin, and examined under a microscope to determine types and quantity of defects. The results showed reduction in the number of bubbles (as well as in void content) at frequencies between 10Hz and 50Hz for 30 minutes of vibrations.