Accordingly, the preferred embodiment of a hydraulic chuck assembly of the present invention is to be provided on one end of a shaft of a lathe and comprises: a hollow main chuck body having a front head being formed with a plurality of radially extending and angularly spaced seat retaining slide grooves; a tubular plunger movably provided inside the main chuck gradually expanding and forwardly extending funnel-shaped cam portion extending into the front head; and a plurality of grip seats each being slidably retained in a respective one of the slide grooves, each of the grip seats having a rear side formed with an inclining groove, said inclining grooves of the grip seats cooperatively receiving the funnel-shaped cam portion, said plunger being movable forward or rearward, and said funnel-shaped cam portion cramming the inclining grooves of the grip seats so as to move the grip seats toward or away from each other along the slide grooves.