AbstractThe magneto‐optical properties of various transparent plastic materials have been measured. It has been clarified that the Verdet constants of polystyrene derivatives are larger than those of NaCl crystal and the light flint glass, and that the value of the figure of merit reaches a maximum at the short wavelength region below 500 nm.An optical isolator using poly‐α‐methylstyrene gives an isolation ratio of 20 dB at 488 nm, showing, for the first time, a possibility of a low‐cost plastic optical isolator. The magneto‐optical properties of these plastics also enable the optical measurement of high magnetic field or hi