Abstract: A clamping pen composed of a clamping tube receiving a core rod member and a clip member extending outward from the clamping tube. The clamping tube has an upper open end and a lower end. The upper end is formed with multiple annular projections and two lateral axially extending slots or fissures. A flange is formed on the clamping tube below the clip member, serving as a fulcrum, whereby when the clip member is pressed down or released, the fissures are expanded or contracted, permitting the rod member received in the clamping tube to be easily moved outward or inward to adjust a protruding length of the rod member. After the adjustment is completed, the clip member is released and the annular projections of the clamping tube are tightly held to contract the fissures and tightly clamp the rod member for easy use.