The behavior of structural OH in clinopyroxene (Cpx) and orthopyroxene (Opx) during successive heating, cooling and stepped heating was investigated by in-situ Micro-FTIR measurements under temperatures ranging from 25℃ to 500℃at 100℃ increments. The results suggest that both Cpx and Opx exhibit a decrease in OH stretching vibration frequency with increasing temperature, and the variation is reversible. The shift in absorption band frequency is controlled by thermal expansivity of the OH site or lattice interstice.