Introduction: Mesenchymal Stem Cells (MSCs) are multipotent cells capable of duplication and auto-recovery and distinction from various cells including chondrocytes, adipocytes, chondroblasts, fibroblasts, and osteoblasts. Human stem cells are always subject to local and external mechanical loads. External loads are caused by physical activity in external environment loading to infliction of static and dynamic loads on the body and internal loads are typically caused due to body physiological function. Mechanical factors can affect different parameters such as morphology, proliferation, migration, metabolism and death as well as chemical changes in cells and lead to chemical changes in extracellular matrix and intracellular environment, besides distinction of cells. Methods: MSCs were isolated from rat’s bone marrow, then cultured in microgravity conditions. Morphologic changes of cells were analyzed by taking pictures at different times. Results: Results indicated a reduction in cell area and an increase in cell aspect ratio, in microgravity conditions. No significant difference was observed in cell angle of rotation at different time measurements. Also, in measuring viability of these cells using MTT test it was found that microgravity reduces viability of stem cells, considerably. Conclusion: Microgravity conditions have a considerable impact on morphology of MSCs. Furthermore, viability of MSCs decreased signi ficantly after 48 h, under microgravity conditions.