Objective To observe the changes of diameter of intrafusal fibers and capsule at equatorial region of rat soleus muscle spindle caused by hindlimb-unloading.Method Tail-suspended female rats were used as the model of hindlimb-unloading,all rats were randomly divided into 4 groups according to body weight.The four groups were exposed to tail-suspension for: 7 d,14 d,28 d and 0 d,respectively.The diameter of intrafusal fibers and capsule of single muscle spindle was measured and analyzed.Result The diameter of intrafual fibers,including nuclear-bag fibers and nuclear-chain fibers, were reduced slightly in both 7 d group and 28 d group and obviously reduced in 14 d group.The capsule diameter at equatorial region was obviously reduced in 7 d group.Conclusion Hindlimb-unloading can obviously reduce the diameters of intrafusal fibers and capsule at equatorial region of rat soleus muscle spindle.