Pairs trading has been popular as a statistical arbitrage among major investment banks and hedge funds since 1980s. Despite the high average return, the idea behind the strategy is simple. If two stocks moved together in the past, they are likely to continue the trend. Therefore, divergence in price triggers a trade. While pairs trading has been used for many years, there is no earlier attempt to optimize the trigger. Therefore, the main objective of this research is to optimize pairs trading strategy. We also provide a profitability indicator measuring theoretical return of pair, by which investors can identify more profitable pairs.