A casing assembly includes a first casing and a second casing. The first casing includes a first lateral plate and a first engaging element. The first engaging element includes a first slab, a first sidewall and a second sidewall. The first sidewall and the second sidewall are connected with the first lateral plate. Moreover, a dent is formed between the first lateral plate and the first engaging element. The second casing is combined with the first casing. The second casing includes a second lateral plate and a second engaging element. The second engaging element includes a second slab, a third sidewall and a fourth sidewall. The third sidewall and the fourth sidewall are connected with the second lateral plate. The second engaging element is accommodated within the dent and in contact with the first engaging element, so that the first casing and the second casing are combined together.