A fencing system includes two spaced-apart post assemblies each including a post member having a bottom receiving space, a core element inserted into the bottom receiving space, and an adjustable press plate disposed transversely on and pressing the core element against a mounting surface. The core element has a through hole extending in top and bottom ends thereof. A fixing bolt has a top end portion extending into the through hole and the press plate. A locking element is connected to the top end portion, and is movable toward or away from the press plate so as to press the press plate against the core element or to release the press plate. At least one of the press plate and the core element is movable transversely of the fixing bolt so that a distance between the post members of the post assemblies can be adjusted.