Handlebar assembly

A handlebar assembly suitable to be installed on a steerer tube of a bicycle, the handlebar assembly comprises:

• a stem having a first end and a second end opposite to the first end, wherein the stem has a first fastening surface positioned at the first end, and the second end is suitable to be assembled to the steerer tube;
• a handlebar comprising a middle portion and two handle portions, wherein the handle portions are positioned on two opposite sides of the middle portion respectively, and the middle portion has a second fastening surface to be infixed to the first fastening surface;
• and
• a fastener crossing the first fastening surface and the second fastening surface infixed to each other to fasten the stem and the handlebar and positioned within the stem and the middle portion, wherein the stem has a convex fastening portion positioned on the first fastening surface, the middle portion has a concave fastening portion positioned on the second fastening surface, and the convex fastening portion is infixed to the concave fastening portion.