Thermoplasticity of two different coals and their blends was investigated upon isothermal and non-isothermal heating at temperatures of 600-800 K. Changes in viscosity (eta) and the fraction of liquid (phi(L); defined as that of mobile hydrogen) of softening coal in a form of disk were successfully measured by means of a variable-force-loading needle penetrometry coupled with a volumetric dilatometry and a proton nuclear magnetic resonance, respectively. eta and phi(L) changed with the temperature and time over ranges of 10(4)-10(10) Pa s and 0.03-0.53, respectively, depending upon the blending ratio and also the heating mode. For every single-coal/blend, a single linear relationship between log eta and phi(L), i.e., log eta = a - b phi(L), was confirmed over the temperature range and regardless of the heating mode, which indicated that the softening coal behaved as a mixture of liquid and "deformable" solid. It was found that both the parameters a and b were linear functions of the blending ratio, and the log eta-phi(L) relationships for the blends were thus predictable from those for the single coals. Furthermore, log eta of the blends was reasonably approximated over the temperature/viscosity ranges only from those of the single coals by assuming additivity of log eta.