We have examined effect of foreign gas and its pressure upon etch rate, surface morphology, and surface chemical composition. Etch rate is suppressed by adding inert gases such as argon and helium. On the contrary, oxygen enhances the rate by increasing its pressure, indicating that oxygen molecules react with ablated polymer surface. From the results of XPS analyses, active surface of ablated polymer reacts also with ammonia and hexamethyldisilane. It is considered that laser ablation has a high potentiality of simultaneous microfabrication and photochemical surface modification of polymer.