The paper studies a pendulum isolator. This isolator has a spherical surface which is placed on the foundation and it has an arm which is connected to the column of the building as a hinge. When the height of the arm is less than the curvature radius of the spherical surface, and a small movement is applied on it, a restoring force will be applied to the isolator. In this paper the effect of horizontal and vertical component of seismic load was studied. The dynamic equation of motion was obtained for the pendulum isolator under horizontal and vertical accelerations of seismic loads. The equation was solved by step time integration method. The comparison of the results of horizontal component effect and the combined effect of horizontal and vertical components on the isolator showed that the vertical component effect increased the isolator horizontal displacement response and increased the isolator horizontal acceleration.Keywords base isolation · vibration control · pendulum isolator · earthquake vertical acceleration effect · combined horizontal and vertical components effects of earthquake