Magnetic properties of organic radical crystals, 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) and 3-(4-bromophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-BDTV), have been studied by measuring the heat capacity and ac susceptibility above 0.13 K in external magnetic fields of 0-30 kOe, p-CDTV has been found to be a new ferromagnet with a Curie temperature of T-c = 0.68 +/- 0.02 K. p-CDTV behaves as a quasi-one-dimensional Heisenberg ferromagnet with the intrachain exchange interaction of 2J/k(B) = +12.0 +/- 0.6 K above the transition temperature T-c. The interchain exchange interaction zJ’/k(B) was estimated to be +0.21 +/- 0.02 K, where z is the number of interchain bonds per spin, p-BDTV, on the other hand, behaves as a one-dimensional Heisenberg antiferromagnet with negative exchange interaction of 2J/k(B) = -41.2 K.