A lens assembly includes a first lens and a second lens. The first lens includes a first inner end surface, a first outer end surface and slots. The slots formed on the first inner end surface extends toward the first outer end surface, and each slot includes a trough and an inner engaging side portion. The second lens includes a second inner end surface, a second outer end surface and engaging blocks. The engaging blocks formed on the second inner end surface extends toward the first inner end surface of the first lens, and each engaging block includes a crest portion disposed next to the first inner end surface of the first lens and an inner engaging protrusion. A slit is defined by the inner engaging side portion and the trough portion of the slot and the inner engaging protrusion and the crest portion of the engaging block.