A fiber loop mirror sensor with a long period fiber grating (LPFG) insc ribed in the polarization maintaining fiber (PMF) is proposed and experimentally demonstrated for measurement of relat ive humidity and temperature simultaneously.The LPFG is inscribed on PMF by using a high-frequency CO_2las er and then connected with a 3-dB coupler to form a fiber optic mirror.The prop osed sensor is fabricated through coating a thin layer of polyvinyl alcohol (PVA) onto the surface of PM-LPFG,which can change it refractive in dex to achieve environmental humidity parameters.The resonant wavelength varies with temperature and its int ensity changes with relative humidity,so relative humidity and temperature can be measured simultaneously.The experimental results show that the achieved sensitivities to relative humidity and temperature are 0.17nm/%RH and 0.21nm/℃,respectively.