Abstract: Blockage effects of an isolated rectangular tall building are investigated in the TJ-2 Boundary Layer Wind Tunnel in Tongji University using the synchronous wind pressure measurement technique. Pressure tests of rigid models with blockage ratios of 4.1%, 6.1%, 8.4% and 10.1% are conducted in the low turbulent uniform flow and results are compared and analyzed. Blockage effects on mean pressure characteristics on model surfaces are emphasized. The results show that in the uniform flow blockage effects on mean pressure coefficients on windward surface of the model are slight and less than 5% except for the corners. Blockage effects reduce mean pressure coefficients on side, leeward surfaces and roof obviously, while the distributions of mean pressure coefficients are almost unchanged. In addition, a new correction formula for mean pressure coefficients is proposed based on the test results.