An assembly for the endoscope is composed of a tubular member, a positioning ring, and a sleeve. The tubular member on its external wall includes a first threaded portion and a second threaded portion spaced from the first threaded portion. The positioning ring includes a threaded part formed on an internal wall thereof for axial movement along the tubular member. The sleeve includes a threaded section formed on an internal wall thereof. When the sleeve is sleeved onto the tubular member, the threaded section is located between the first and second threaded portions and stopped against the first threaded portion; meanwhile, the positioning ring can be pushed against the sleeve to fasten the sleeve onto the tubular member. Accordingly, the sleeve can be accurately, rapidly, and conveniently positioned onto the tubular member.