A sprinkler includes a hollow shaft to which a positioning part, a gear casing, a pipe are respectively connected to the hollow shaft. The positioning part and the pipe are respectively connected to a first end support and a second end support of a base. The positioning part has multiple grooves defined in a tubular portion thereof. A hollow positioning rod has a ratchet portion which is resiliently formed thereto. The hollow positioning rod is rotatably mounted to the hollow shaft and the ratchet portion is engaged with the grooves of the positioning part. The hollow positioning rod has a toothed portion which is engaged with an output gear of the gear casing. The gear casing is driven by a hydraulic power so as to swing the pipe periodically. The resilient ratchet portion allows the gear casing and the pipe to swing easily, and no metal spring is required.