Magnetic property of 3-(4-cyanophenyl)-1,5-diphenyl-6-thioxoverdazyl (p-CyDpTV) radical crystal has been studied in the temperature range of 0.05-300 K in external magnetic fields 3, 11, 30, and 100 Oe by using SQUID magnetometer. Weak ferromagnetic behavior was observed at T N = 0.41 K. Below T N , the spontaneous magnetization M s (T) increased rapidly, reaching a maximum of 31.1 emu/mol at 0.05 K. Spin-canting angle was estimated to be 0.32° at 0.05 K. Near T N , m s (T) [1 - (T/T N )] β with the critical exponent of β = 0.36, in agreement with that expected for an isotropic three-dimensional (3D) Heisenberg spin system.