This study was an attempt to prove that the thickness of the outer egg membrane is the key factor that contributes to the normal zoeal larvae hatching rate of the in vitro artificially fertilized eggs of the Japanese mitten crab Eriocheir japonica by artificially stretching and thinning the outer egg membrane with the help of water surface tension. The results showed that the artificially thinned outer egg membrane became six times thinner. In addition, the normal zoeal larvae hatching rate of the in vitro artificially fertilized eggs rose from 10% to over 66% after the thinning treatment. Discussion focuses on the mechanism of this thinning phenomenon, the role the thinned outer egg membrane plays in the increase of the normal zoeal larvae hatching rate of the in vitro artificially fertilized eggs and the utilization of this particular outer egg membrane thinning method. (C) 2011 Elsevier B.V. All rights reserved.