ZnO and ZnO: Mn nanocrystals were synthesized via reverse micelle method. The structural properties of nanocrystals were investigated by XRD. The XRD results indicated that the synthesized nanocrystals had a pure wurtzite (hexagonal phase) structure. Resistive gas sensors were fabricated by providing ohmic contacts on the tablet obtained from compressed nanocrystals powder and the installation of a custom made micro heater beneath the substrate. Sensitivity (S= Ra/Rg) of ZnO and ZnO: Mn nanocrystals were investigated as a function of temperature and concentration of ethanol and gasoline vapor. The obtained data indicated that optimum working temperatures of the ZnO and ZnO: Mn nanocrystals sensors are about 360°C and 347°C for ethanol vapor and about 287°C and 335°C for gasoline vapor. Based on gas sensing results, although Mn impurity reduces the Sensitivity but the sensor got saturated at much higher gas concentration.