A telescopic handwriting pen for PDA or cellular phone. In one embodiment, the pen comprises tubelike inner, intermediate, and outer sections and first and second coupling assemblies fitted in front ends of the inner and intermediate sections. Either coupling assembly includes annular grooves with O-rings put thereon, an annular recess between the grooves filled with lubricating oil, and a rear extension secured to the inner or intermediate section. A writing head is fitted in a front end of the outer section and comprises a channel in fluid communication with the external and inside of the second coupling assembly. Stop mechanisms are provided in rear ends of the outer and intermediate sections for preventing an inside section from disengaging another section while sliding therein. The pen can carry out a smooth sliding and has a balanced weight.