Quantum statistics for the most isotropic spin (S=1/2) system in one dimension has been studied with a newly synthesized ferromagnetic organic radical crystal of 3-(4-chlorophenyl)-1,5-diphenyl oxoverdazyl. The characteristic field dependence of the magnetic heat capacity of this crystal is quantitatively explained by the theoretical values derived from the Betheansatz technique applied to the quantum transfer matrix, with a unique intrachain exchange constant J/k(B)=5.5 It. By the simultaneous measurement of magnetic susceptibility and heat capacity, the three-dimensional ordering at T-c=0.21 K, which is triggered by a weak interchain coupling zJ’/k(B) = 3 x 10(-2), is confirmed.