Heat capacities of the organic free radical ferromagnet 4-benzylideneamino-2,2,6,6-tetramethylpiperidin-1-oxyl (BATMP) crystal were measured in the temperature range between 0.1 and 300 K by adiabatic calorimetry. A ferromagnetic phase transition was found at T-C = 0.19 K, and a broad heat-capacity anomaly was found arising from the short-range ordering above T-C characteristic of low-dimensional magnetic spin systems. The enthalpy and entropy gains due to both the magnetic phase transition and the heat-capacity anomaly were evaluated to be DeltaH = 3.86 J mol(-1) and DeltaS = 5.64 J K-1 mol(-1), respectively. The value of the experimental magnetic entropy agrees well with the theoretical value R ln 2 (5.76 J K-1 mol(-1)) expected for a spin quantum number S = 1/2 spin system (R is the gas constant). The magnetic heat capacity hump due to the short-range order was well accounted for in terms of an S = 1/2 one-dimensional ferromagnetic Heisenberg model with the intrachain exchange interaction J/k(B) = 0.95 K (k(B) is Boltzmann's constant). This fact suggests that BATMP crystal is a one-dimensional ferromagnet above T-C. The spin wave analysis of the magnetic heat capacities below T-C revealed that BATMP crystal is in a three-dimensional ferromagnetic state below T-C and the averaged interchain exchange interaction is J'/k(B) = 0.026 K.