An improved structure of writing instrument includes a pen body including a pen barrel and an ink refill and tip mounted in the pen barrel, a positioning device including a retaining ring mounted in the pen barrel and a positioning protrusion located at the retaining ring and extended outside the pen barrel, and a pen clip having a clickable top end thereof coupled to the pen body that controls the ink refill and tip between an extended position and a retracted position and an opposing bottom end thereof provided with a U groove that is coupled with the positioning protrusion of the positioning device to guide the pen clip to slide up and down in relation to the positioning protrusion stably and smoothly, and prevent deviation or loosening.