The magnetic properties of 4-methacryloylamino-2,2,6,6-tetramethylpiperidine-1-oxyl (MATMP) have been investigated by means of adiabatic heat capacity calorimetry in the 0.08-24 K range. The heat capacities of MATMP showed a magnetic phase transition at 0.15 K and a hump centered around 1 K, arising from the short-range ordering characteristic of low-dimensional magnets. The entropy gain due to these two magnetic thermal anomalies was Delta S = 5.49 J K-1 mol(-1). The hump can be well accounted for in terms of the spin S = 1/2 one-dimensional ferromagnetic Heisenberg model with an intrachain exchange interaction of J(parallel to)/kB = 0.70 K, where k(B) is the Boltzmann constant. The temperature dependence of the magnetic heat capacities below the transition temperature is approximated by T-3/2. This fact, on the basis of the spin-wave theory, implies that MATMP would be a ferromagnet below the transition temperature. However, the magnetic susceptibility measurement reported recently suggests that MATMP would be antiferromagnetic below the transition temperature. A possible origin responsible for this contradictory feature is discussed.