Disclosed is a novel negative-working chemical-amplification photoresist composition comprising (A) an alkali-soluble resin, (B) an acid-generating agent and (C) a crosslinking agent, of which the component (B) is an onium salt compound selected from the group consisting of iodonium salt compounds and sulfonium salt compounds, having a specific fluoroalkyl sulfonate ion as the anionic moiety and the component (C) is a specific ethyleneurea compound substituted for at least one nitrogen atom by a hydroxymethyl or alkoxymethyl group. The photoresist composition is particularly suitable for the formation of a photoresist layer on a substrate surface provided with an undercoating of a water-insoluble organic anti-reflection film exhibiting excellent pattern resolution and orthogonal cross sectional profile of the patterned resist layer with a good temperature latitude in the post-exposure baking treatment for latent image formation.