The high-temperature cell parameters of arsenopyrite (FeAsS) have been determined from 300 to 673 K through in situ angle-dispersive X-ray diffraction experiments with synchrotron radiation at Beijing Synchrotron Radiation Facility (BSRF). No phase change has been observed up to the maximum temperature in this study. Thermal expansion coefficients of arsenopyrite were determined from the measured cell volumes with equations of the form alpha= alpha(0) + alpha T-1+alpha(2)/T(2)for the first time. The value of volume expansion coefficient is 3.1(2) x 10(-5) K-1. Furthermore, we confirm that the linear expansivities along a, b and c directions of arsenopyrite are almost isotropic [alpha(a) = 1.2(3) x 10(-5), alpha(b) = 0.8(3) x 10(-5), and alpha(c), = 0.8(2) x 10(-5) K-1] within their uncertainties. Furthermore, the effects of anionic substitution on the determination of expansivity between arsenopyrite and pyrite were discussed.