An adjustable fixture for clamping a tube-like object therein regardless of variation in the dimension, comprising: a lower clamp piece of generally semi-circle shape having first and second lateral sides (4,5) opposite to each other; an upper clamp piece of generally semi-circle shape having third and fourth lateral sides (7,8) opposite to each other, wherein the third lateral side is connected pivotally to the second lateral side of the lower clamp piece such that the upper clamp piece is pivotable relative to the lower clamp piece between a closed position, where the upper clamp piece covers the lower clamp piece to define a clamping space therebetween, and an open position, where the lower clamp piece is uncovered by the upper clamp piece, the upper clamp piece further having an opposing hook edge projecting outwardly from the fourth lateral side; a torque rod having a first end pivoted to the first lateral side of the lower clamp piece; a traction element having a first traction end pivoted to the torque rod adjacent to the first end thereof and a second traction end that is opposite to the first traction end and that defines a threaded through hole, the traction element including a threaded bolt disposed threadedly and movably in the threaded through hole; and a hook disposed on the second traction end of the traction element and operably associated so as to be movable together with the threaded bolt in such a manner to engage the hook with the opposing hook edge of the upper clamp piece when the upper clamp piece is disposed in the closed position, thereby preventing the upper clamp piece from pivoting back to the open position, characterised in that the second traction end of the traction element defines three threaded through holes respectively receiving three threaded bolts therein, each of the threaded bolts having a bolt head, and in that three threaded nuts respectively are threadedly fastened to the threaded bolts while the hook defines three sleeve holes respectively sleeved around the corresponding threaded bolts in such a manner that the hook is secured between adjacent pairs of the bolt heads and the threaded nuts, thereby preventing wobbling of the hook relative to the threaded bolts and providing additional engagement of the hook with respect to the threaded bolts. a lower clamp piece of generally semi-circle shape having first and second lateral sides (4,5) opposite to each other; an upper clamp piece of generally semi-circle shape having third and fourth lateral sides (7,8) opposite to each other, wherein the third lateral side is connected pivotally to the second lateral side of the lower clamp piece such that the upper clamp piece is pivotable relative to the lower clamp piece between a closed position, where the upper clamp piece covers the lower clamp piece to define a clamping space therebetween, and an open position, where the lower clamp piece is uncovered by the upper clamp piece, the upper clamp piece further having an opposing hook edge projecting outwardly from the fourth lateral side; a torque rod having a first end pivoted to the first lateral side of the lower clamp piece; a traction element having a first traction end pivoted to the torque rod adjacent to the first end thereof and a second traction end that is opposite to the first traction end and that defines a threaded through hole, the traction element including a threaded bolt disposed threadedly and movably in the threaded through hole; and a hook disposed on the second traction end of the traction element and operably associated so as to be movable together with the threaded bolt in such a manner to engage the hook with the opposing hook edge of the upper clamp piece when the upper clamp piece is disposed in the closed position, thereby preventing the upper clamp piece from pivoting back to the open position, characterised in that the second traction end of the traction element defines three threaded through holes respectively receiving three threaded bolts therein, each of the threaded bolts having a bolt head, and in that three threaded nuts respectively are threadedly fastened to the threaded bolts while the hook defines three sleeve holes respectively sleeved around the corresponding threaded bolts in such a manner that the hook is secured between adjacent pairs of the bolt heads and the threaded nuts, thereby preventing wobbling of the hook relative to the threaded bolts and providing additional engagement of the hook with respect to the threaded bolts.