PURPOSE: To obtain the titled compsn. whose ink passage duct has good sectional shape reproducibility, shape stability and mechanical characteristics and which has persistent smoothness in writing and is suitable for use in writing fine letters, by incorporating a nucleating agent in a polyoxymethylene homopolymer.

CONSTITUTION: A compsn. for use in forming the body of a plastic pen is obtd. by incorporating a nucleating agent in a polyoxymethylene homopolymer. The polyoxymethylene homopolymer which can be used has a number-average MW of 50,000W200,000 and is formed by polymerizing formaldehyde or a cyclic oligomer of formaldehyde and stabilizing the polymer against decomposition from its terminal. The nucleating agent to be incorporated in the poly oxymethylene homopolymer may be any of inorg. nucleating agents and org. nucleating agents. Examples thereof are antimony trioxide, boron nitride, pyrophyllite, fluororesin powder, urea derivative powder, etc.