Abstract: The square inverted nine-spot was divided into two symmetric fiow units according to its symmetry so as to calculate the water fiood sweep efficiency of heavy oil reservoirs in inverted nine-spot pattern accurately and quickly and evaluate the well pattern effectively. With the non-Darcy fiow as the beginning, the formula for calculating the areal sweep efficiency of ordinary heavy oil reservoir by non-piston like water fiood in square inverted nine-spot pattern was derived by means of fiow line integral method. In this formula, both threshold pressure gradient and oil/water viscositу ratio are taken into account. Then, this formula was used to analуze the effects of threshold pressure gradient and injection/production parameters on the areal sweep efficiency of ordinary heavy oil reservoir in square inverted nine-spot pattern. Finallу, it was applied to evaluate the existing well pattern in NB conventional heavу oil reservoir in Kazakhstan, and the corresponding well pattern adjustment method was proposed. It is indicated that the areal sweep efficiency of con-ventional heavу oil reservoir can be increased effectivelу bу decreasing the well spacing or increasing the injection/production pressure difference, and the effects of injection/production pressure difference and well spacing on the areal sweep efficiency get more obvious with the increase of threshold pressure gradient of oil reservoir.