The thermal expansion of a ferromagnetic solid solution of (Mn1−xTax)3B4was determined using a high temperature x‐ray powder diffraction technique in the temperature range 300–973 K. From the thermal expansion curve, the values of the exchange striction along three axes were deduced. They were negative along bothaandbaxes, but the thermal expansion of thecaxis was very small and exchange striction was not observed. It was considered that such an anisotropy in the thermal expansion of the present sample was due to the crystallographic structure of Ta3B4. The pressure dependence of the Curie temperature of (Mn0.8Ta0.2)3B4was determined to be 0.77 deg/kbar from electrical resistivity measurements under high pressure. The observed value ofdTc/dpis in good agreement with the value calculated using the molecular theory proposed by Bean and Rodbell.