The object of this paper is to develop a conceptual design sizing tool for earth observation microsatellites. This sizing tool can be used to get conceptual but useful estimation for some design parameters which are considerable for LEO (Low Earth Orbit) earth observation microsatellites. The satellite is assumed to have a LEO SS-O (Sun Synchronous Orbit), because this orbit type has desirable advantages for remote sensing and earth observation missions. In addition, the observation mission is assumed to be at satellite nadir direction. The sizing tool is developed based on both mission and system design-estimating relationships. In addition, the conceptual design data and concepts of the microsatellites with similar missions have been used to attain a reliable sizing tool supported with logical assumptions. Design parameters incorporated with the sizing tool include mission requirements such as revisit time as well as subsystems parameters like the type of solar cells and battery cells which can be used in electrical power subsystem. It has been assumed that the type of the earth observation payload is Multi-spectral Mid-IR with the purpose of natural resources remote sensing. Finally the influence of these factors on the total mass of the satellite has been investigated through the sizing tool output diagrams.