For porous membranes, membrane porosity determines permeability whereas membrane pore size determines selectivity. In many cases, increasing membrane porosity can elevate the permeability but often sacrifices the selectivity due to the accompanying increase of pore sizes. In this study, an in-situ assembly process is incorporated into the membrane formation process to manipulate membrane surface porosity and pore sizes. Amphiphilic copolymer, Pluronic F127, is utilized as a pore-forming agent in the casting solution, and tannin (TA) is utilized as an additive in the coagulation bath. During the membrane formation process, the in-situ assembly of Pluronic F127 and TA via hydrogen bonding interaction can disturb the nucleation and growth process of the membrane formation, increasing membrane surface pore density and decreasing membrane surface pore sizes simultaneously. With the increase of TA content from 0 wt% to 6 wt%, the membrane pore density is increased from 18 to 61 every square micron, and the surface pore sizes are decreased from 17.4 +/- 1.4 to 9.3 +/- 0.4 nm. The surface porosity, as a comprehensive result of the surface pore density and sizes, is increased from 1.87 +/- 0.02% to 2.89 +/- 0.17%, then decreased to 1.67 +/- 0.19%. The membrane with the TA content of 2 wt% exhibits high separation performance with the initial flux of 412 +/- 29 Lm(-2) h(-1) bar(-1), and the distinguished rejections for bovine serum albumin and dextran (150 kDa) of 99.0 +/- 0.1% and 90.9 +/- 0.6%.