A positive resist composition comprises a polymer having a carboxyl group substituted with an acid labile group having formula (1) wherein R1 and R2 are alkyl or alkenyl, R3 and R4 are a single bond, methylene, ethylene or propylene, R5 and R6 are hydrogen or alkyl. The composition has a high dissolution contrast, high resolution, and suppressed acid diffusion rate, and forms a pattern of good profile and minimal edge roughness.