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Influences of relative humidity and temperature on PMMA based fiber Bragg gratings (FBGs) are systematically studied. First, we have found that relative humidity exhibits much significant influence on the Bragg wavelength. An increase of 1% in relative humidity leads to a shift of 66 pm of Bragg wavelength centered at 1301 nm at 20°C, while 1°C increase in temperature only results in -7.8 pm shift for the same grating at constant 60%RH. Secondly, a strong synergetic effect has been identified between temperature and relative humidity-temperature sensitivity of PMMA based FBGs is humidity-dependent and humidity sensitivity is temperature-dependent. Finally, taking into consideration of this synergy, the temperature sensitivity of three Bragg gratings in a single fiber with central wavelength of 957, 1304 and 1545 nm peak are determined as -4.3, -7.6 and -8.3 pm/°C, respectively, at constant 60%RH. These sensitivity values are one order of magnitude lower than those measured without controlling ambient humidity.