The effects of natural convection instability on ultrafiltration performance have been tested experimentally in empty (without spacer) and spacer-filled channel cells. In dead-end operation, the permeate fluxes at the gravitationally unstable orientation of the empty cell are enhanced up to 3.5 times for dextran solution and 5.5 times for BSA solution compared with the results at the stable orientation. In cross-flow operation, flux improvement by natural convection instability occurs when the cross-flow velocity is below the critical value of around 0.1-0.2 m/s (Re = 35-90). A general criterion for determining whether natural convection effects dominate is identified considering the mass transfer between the membrane surface and the bulk. The criterion is that when Gr/Re-2 > ca. 3 (for the empty cell) or Gr/Re-2 > ca. 500 (for the spacer-filled cell) natural convection instability is of importance. A mass transfer correlation for the mixed convection membrane system is presented.