Abstract: Both theoretical and experimental studies of the substrate effect on the thermal behavior of a PbTiO/sub 3/ infrared (IR) sensor have been reported. With active cantilever dimensions of 200/spl times/100/spl times/5 /spl mu/m/sup 3/ formed by etching processes, the pyroelectric micro-electro-mechanical system (pyro/MEMS) structure exhibits a much superior performance to that of a traditional IR-sensing bulk structure under the 800-/spl mu/W incident optical light with wavelength of 970 nm. Two-order improvement in current responsivity is obtained for the pyro/MEMS structure. This shows the substrate effect on the performance of a pyro/MEMS IR sensor is very significant. A simple model has also been proposed to illustrate the substrate effect more comprehensively.