The phase control characteristics in a thyristor phase control circuit with parallel resonance elements indicate very interesting phenomenon. Several extreme values appear on the phase control curve. The phenomenon is different from the step-up one in a thyristor phase control circuit with series RLC elements which is interpreted as series resonance. To comprehend the circuit performance with those loads, it is necessary that the phenomenon on extreme value is physically clarified from other viewpoints. In this paper the performance in this circuit is studied from two viewpoints of a natural oscillation and a parallel resonance. Then, it is found that the performance depends on a natural frequency in thyristor conducting period and a parallel resonance frequency in thyristor non-conducting period. Therefore, the interesting phenomenon on extreme value is affected by the alternative of natural frequency or parallel resonance frequency.