An optical assembly, a backlight module and a liquid crystal display are provided. The optical assembly includes a light guide plate and a frame. The light guide plate has at least one first circular-arc structure disposed on a side of the light guide plate. The frame is disposed around the light guide plate, in which the frame has at least one second circular-arc structure interlocked with the first circular-arc structure. A radius of the first circular-arc structure is substantially equal to a radius of the second circular-arc structure.