A photosensitive resin composition and a method of forming a positive resist pattern by the use of the composition are disclosed. The photosensitive resin composition comprises (A) a resinous compound containing an acid-decomposing ester group, (B) a compound containing one ethylenically unsaturated bond in its molecule and possessing a group capable of forming a carboxylic acid via decomposition by the action of an acid, (C) a photo-acid generator, (D) a photo-radical polymerization initiator, and optionally (E) an epoxy resin. When this composition is applied to a substrate, exposed to an active energy ray of a wavelength enough to activate the photo-radical polymerization initiator (D) and not enough to activate the photo-acid generator (C) to radically polymerize the compound (B), selectively irradiated with an active energy ray of a wavelength enough to activate the photo-acid generator (C) and heated, the resinous compound (A) and the polymer of the compound (B) is thermally decomposed by the catalytic action of the acid generated by the photo-acid generator (C), with the result that the selectively irradiated part of the coating film will be rendered soluble in an aqueous alkaline solution. A positive resist pattern is formed by removing the selectively irradiated part by development. When the composition contains the epoxy resin (E), the coating film may be thermally cured.