Abstract: An anti-reverse transmission for use in a window blind includes a fixed axle, a rotatable input shaft, an output shaft sleeved onto the fixed axle, a transmission shaft sleeved onto the output shaft and coupled to a linking rod of a window blind, the transmission shaft having a protrusion inserted into an opening in the output shaft, and a spiral spring fitted on the fixed axle within the output shaft. The spiral spring has two end tips suspended in the opening such that the spiral spring is radially expanded to disengage the output shaft from the fixed axle for rotation with the transmission shaft and the input shaft upon rotation of the input shaft, and the spiral spring is radially compressed to stop the output shaft from rotation when the transmission shaft receives a biasing force from the gravity weight of a blind body of the window blind.