A fitting (1) for a plastic pipe (11) which is capable of maintaining a uniform seal ability regardless of deformation of pipe owing to a temperature change and aging. The fitting (1) has a fitting body (2) having an outer thread (7) and an insertion part (6) to be inserted into a pipe (11), a brass slite olive (4) adapted to be inserted on the insertion part (6) and in contact with an end face of the fitting body (2), and a tightening nut (3) having an inner thread (3a) adapted to be engaged with the outer thread (7) of the fitting body (2), the insertion part (6) of the fitting body (2) being formed with a plurality of annular grooves (6a) having rectangular sections at its outer surface and the annular grooves (6a) being fitted with rubber 0-rings (5).
