In this paper, theoretical analysis is presented on predistortion linearization of the optical fiber AM CATV externally modulated transmitter, and a circuit is designed based on Schottky diodes. Experimental results show that this predistortion circuit suppresses the third order harmonic (3HD) at least 13 dB from 50 MHz to 200 MHz, and has no effect on the second order harmonic (2HD) in the same time.