Abstract: A femur size measuring device include a stylus part including a stylus arm and a stylus holder, an anterior posterior (AP) sizer part having the stylus part coupled to an upper end, a pair of paddles at a lower portion, and an anterior pin guide mount and a posterior pin guide mount, a plurality of anterior pin guides mounted to be replaceable in the anterior pin guide mount, configured to determine insertion positions of a pair of anterior position determination pins for determining a rotation position of a femur cutting guide, and configured to guide the anterior position determination pins, and a plurality of posterior pin guides mounted to be replaceable in the posterior pin guide mount, configured to determine insertion positions of a pair of posterior position determination pins for determining a rotation position of a femur cutting guide, and configured to guide the posterior position determination pins.