In view of the incapability of the current evaluation method of primary frequency modulation in accurately reflecting the effect of primary frequency modulation， an evaluation method of primary frequency modulation effect based on the characteristics of unit output power curve variation is proposed. The characteristics the unit primary frequency modulation should have are discussed， and based on this concept， four indices are presented to evaluate the performance of unit primary frequency modulation. Then problems such as the essential factors of the effective disturbance， the impact and treatment method of automation generation control on primary frequency modulation assessment are treated. The proposed method is suitable for power grids with different capacities and different load characteristics， and is applied to the unit primary frequency modulation assessment in Liaoning Power Grid.