We propose an improved implementation of modified Weil pairings. By reduction of operations in the extension field to those in the base field, we can save some operations in the extension field when computing a modified Weil pairing. In particular, computing e (P, φ(P)) is the same as computing the Tate pairing without the final powering. So we can save about 50% of time for computing e (P, φ(P)) compared with the standard Miller’s algorithm. keywords : pairing-based cryptosystem, Weil pairing, modified Weil pairing, separable endomorphism, distortion map