The effects of humidity and temperature on tribological properties of mica and of the octadecyltriethoxysilane (OTE) monolayer self-assembled on mica are studied by friction force microscopy under controlled environments. As the humidity increases, the friction force is observed to decrease for mica and to increase for OTE at low loads at room temperature. Despite their hydrophobicity, water can penetrate into OTE films to alter their molecular chain ordering and to detach the OTE molecules from the mica substrate. The occurrence of OTE film damage is manifested as a crossover point in the friction force curves. Heating the samples in the temperature range of 20-80 degrees C induces a negligible effect for both mica and OTE in low humidity. However, in high humidity, a distinctive change in friction was found. Most of the heating effect can be understood by a "local humidity" concept although various discrepancies still exist.