Weightless environment due to prolonged Space mission results in decreased mineralisation of the weight bearing bones. Hind limb unweighting (HU) in rats by tail suspension was used to simulate the effect of weightlessness on tibia. Adult male albino rats were divided into two groups as (i) Control (CON, n = 12) and (ii) HU for 15 days (HU, n = 18). After 15 days of HU tibia from all the animals were removed and subsequently dried and ashed. The calcium content of these bones were then determined. HU resulted in atrophic changes in the weight bearing bone, tibia, due to the reductions of water content (-35.8%), organic matrix (-12.2%) and calcium content (-33.4%). The reduction in the dry wt of tibia (-13.5%) was due to proportionate reductions in the organic matrix and total mineral content of the bone. The reduction in the mineral content was solely due to the reduction in calcium content of the bone.