The surface dilatational modulus of the monolayer of a well-known semiflexible polymer, poly(n-hexyl isocyanate) (PHIC), at the air water interface was measured as a function of the molecular weight of PHIC. In the dilute regime, the surface dilatational modulus of PHIC showed a linear response over a measured strain range of 5%-20%. The modulus could be well-separated into the elastic component E' and the viscous component E '', of molecular weight of PHIC. The E' and E '' components, respectively, are mainly attributed to static compression modulus caused by a change in the area covered by the PHIC monolayer and the semiflexibility of the PHIC chain. In the semidilute regime, the surface dilatational modulus of PHIC showed a nonlinear response to even 1% of strain due to the semiflexibility of the PHIC chain. Moreover, the surface dilatational modulus of PHIC depended on its molecular weight: at the smaller strain, the surface dilatational modulus of the high molecular weight PHIC was larger than that of the low molecular weight PHIC. The surface dilatational modulus of the high molecular weight PHIC increased with the surface concentration, whereas that of the low molecular weight PHIC remained constant. The molecular weight dependence in the semidilute regime is caused by the difference in chain entanglement between the high and low molecular weight PHICs at the air-water interface.