Aim\ To study energy band structure of short period superlattices and to find out the property of the superlattices to absorb infrared radiations. Methods\ We calculated the band structure of (GaAs) n (AlAs) n under the theory of empirical pseudo potential method and compared difference about energy gap of superlattices. Results\ Calculation indicates that (GaAs) n (AlAs) n is an indirect gap semiconductor when n ≤5. Meanwhile there are large amount of subbands and interbands crossing in superlattices. Conclusion\ Superlattices might be a good material to absorb infrared radiations; similar pseudo potential form factors and different calculating method give the different results.