The characteristic of mutual compensation between Faraday magnetooptical effect and electrogyration effect is experimentally demonstrated by the use of single lead molybdate ( PbMoO4 ，PMO) crystal.The teaching purpose of this experiment is to help college students to understand similar and inherent characteristic of the two optical effects，i.e.，external field-induced polarization rotation in crystals. Experimental setups mainly include two polarizers，one block of single PMO crystal together with its modulating current and voltage supplies. Under simultaneous application of an ac current and an anti-phased voltage to the PMO crystal，we have observed the phenomenon of mutual compensation between the two effects，and the compensating voltage is about 0.541 kV/A.