The effect of membrane inclination on the flux of single-phase or gas-liquid two-phase ultrafiltration in a tubular membrane has been investigated. Experimental result shows that membrane inclination has a significant enhancement on the flux of two-phase ultrafiltration operated at slug flow pattern. As the angle of inclination from the horizontal increases, the flux increases, reaches a maximum, and then decreases. The flux may be enhanced more than 1.5 when the membrane is inclined from 0 to 50 degrees. The flux enhancement due to membrane inclination increases with increasing the gas velocity, the feed concentration, and the transmembrane pressure, while it decreases with increasing the liquid velocity. The optimal inclination angle of the membrane in a slug-flow ultrafiltration is close to 50 degrees. An equation for determining the optimal inclination angle was also proposed in this work.