Abstract : Almost Perfect Nonlinear(APN) S-boxes are used in block ciphers to prevent differential attacks. The non-evidence of permutation APN S-box on even number of variables and the efficiency of power functions bring the importance of power APN S-boxes to use in block ciphers. We present a special class of 3-to-1 S-box on even number of variables. The power APN S-boxes on even number of variables fall in this class. We study some other known APN S-boxes on even number of variables whether they fall in this class. Then we present a necessary condition for power functions to be APN. Using this necessary condition we can filter out some non-APN power functions. Specifically, if the number of variables is multiple of small primes, then one can filter many non-APN functions.