[[abstract]]Mechanical loading is an important factor for normal bone growth, and weight-bearing physical activities are necessary for the skeletal system to achieve sufficient mechanical loading. In swimming, the skeletal system receives its main mechanical loading from muscle contraction. It’s unknown whether different swimming intensities produce different muscle tension on the bone. The purpose of this study was to investigate the effects of different endurance swimming training intensities on skeletal development. Thirty male Wistar rats were randomly assigned into three groups: SWIM 1 group (high intensity), SWIM 2 group (moderate intensity) and a control group. The exercise intensity was increased weekly during the training period. At the end of the training period, animals of the SWIM 1 and SWIM 2 groups swam with a lead weight attached to the tail, which represented 4% and 2% of the animals’ body weight respectively. After 8 weeks of training, the swimming groups showed a significantly lower body weight than the control group (400.2±35.5, 394.8±32.1 <473.1±31.6g) and had higher citrate synthase activity in the extensor digitorum longus (EDL) (6.5±2.4, 6.5±1.5 > 4.9±1.3 mmole/g/min). There was no significant difference in serum bone markers, including total alkaline phosphatase (ALP), carboxyterminal propeptide of type I procollagen (PICP) and carboxyterminal cross-linked telopeptide of type I collagen (ICTP). However, the bone volume ratio of the epiphysis of the SWIM 1 (42±5%) and SWIM 2 (42±4%) groups was higher than the control group (36±3%)（p < .05）. The swim groups showed a significantly lower bone mineral density (BMD) and bone mineral content (BMC) in portions of the tibia and femur. For the bone shape, the swimming groups showed a shorter and more slender tibia and femur. Both swimming groups showed a weaker material property with a lower bending maximal load (ML) on the femur and a lower fracture load (FL) on the femur and tibia. However, the bending stress of the SWIM 2 group (208.6±24.8 Nt/mm2) showed a significantly better mechanical property on the tibia as compared to the control group (150.2±27.0 Nt/mm2). Although there was no significant difference, SWIM 1 showed higher values in bone shape parameters, BMD and material strength. Conclusions: 1) Swimming training causes lower body weight, and seems to negatively affect the absolute bone development (BMD, BMC, bone shape and bone mechanical properties); 2) A higher training intensity might lead to higher muscle tension which could contribute to better bone development.