According to the experiments by Vekilov et al., the crystal growth rate, G, of tetragonal lysozyme crystal initially increases up to 200 mu m/s with increasing flow velocity, u(m) and then decreases monotonically. While in the lower supersaturated solution, G(c) decreases monotonically with increasing u(m) and the growth cessation occurs at u(m) = 1450 mu m/s. In this paper, we propose a new theory to explain these complicated experimental results. We consider one protein molecule near the crystal surface and define a dimensionless number r, the ratio of the velocity of convective flow to the diffusion velocity of a protein molecule. When convection occurs along the crystal surface and a molecule diffuses toward the crystal surface, r indicates the moving direction of the molecule and is closely related with crystal growth rate. We derive the equation to calculate r from experimental parameters. Analyzing the experimental results in the literatures, we find that the dependence of G(c) on u(m) is related with r and there exist three critical r-values, r(1) approximate to 4-8, r(2) approximate to 0.01-0.02, r(3) approximate to 0.004. When r > r(1). crystal growth cession occurs; when r(1) > r > r(2), G(c) decreases with u(m); when r(2) > r > r(3), G(c) increases with u(m); when r < r(3), G(c) is independent of u(m). Though our model is very simple, the dimensionless number r can explain the effects of convection on crystal growth rate consistently. (C) 2008 Elsevier B.V. All rights reserved.