A transmission mechanism of handle lock set used for driving a latch to perform extension/retraction action includes a driving plate and a transmission plate. The driving plate includes a supporting portion having a transmission surface with an arc surface and a blocking surface with a flat surface. The driving plate is rotated to make the supporting portion swinging between a first position and a second position. The transmission plate further includes a protrusion having a contact surface. The transmission surface contacts the contact surface and pushes the protrusion to prevent the driving plate from being unavailable due to transmission dead spot occurrence when the supporting portion locates at the first position. The blocking surface contacts the contact surface of the protrusion to prevent the driving plate from over rotation for avoiding unavailable return when the supporting portion of the driving plate locates at the second position.