A chain is composed of multiple units each having two outer plates, two inner plates and a pin securely connecting the outer plates to the inner plates. The inner plate has two hollow cones oppositely formed on two sides of the inner plate and two tubes each formed on a corresponding one of the hollow cones. A space between an inner face of the hollow cone and the tube is defined to receive therein oil. A rivet hole is defined to align with a pin hole in the outer plate. Therefore, after the outer plates and the inner plates are combined by the pin, the oil inside the chain unit is able to gradually flow out of the chain to lubricate the parts.
