There is provided a photosensitive resin composition for forming an optical waveguide which has high shape precision and excellent transmission characteristics under high temperature and high humidity. A composition of the present invention contains (A) a polymer having structures represented by the following general formulae (1) and (2) (in the formulae, each of R.sup.1 and R.sup.2 is independently a hydrogen atom or an alkyl group having 1 to 12 carbon atoms; R.sup.3is an organic group containing a radical-polymerizable reactive group; X is a single bond or a bivalent organic group; and Y is a non-polymerizable organic group), (B) a compound having at least one ethylenic unsaturated group in the molecule thereof, having a molecular weight below 1,000 and having a boiling point of at least 130.degree. C. at 0.1 MPa, and (C) a photoradical polymerization initiator.