Pairing based cryptography is one of the best security solution that devote a lot of attention. So, to make pairing practical, secure and computationally effcient, we choose to work with extension finite field of the form π½
p
k
with k β₯ 12. In this paper, we focus on the case of curves with embedding degree 18. We use the tower building technique, and study the case of degree 2 or 3 twist to carry out most arithmetics operations in π½
p
2 , π½
p
3, π½
p
6, π½
p
9 and π½
p
18, thus we speed up the computation in optimal ate pairing.