The effect of freeze and thaw cycles on the dynamic modulus and weight change of Nylon fiber concrete was studied in this paper.The change of mechanical properties of Nylon fiber mortar with changing cycle number was also investigated. The test results show that the loss of weight and dynamic modulus of fiber concrete at 300 cycles of freeze and thaw is far less than that of concrete without fiber,and the loss of compression and flexural strength of fiber mortar is also less than that of mortar without fiber.The improvement of freeze thaw resistance is however partly due to the air entrainment properties of the Nylon fiber.