The relative window method provides quantitative crosstalk delay degradation for the post-layout timing analysis in deep sub-micron VLSI design. However, to the best of our knowledge, the relative window method has not been applied to the crosstalk minimization in gridded channel routing problem. Most conventional crosstalk optimizers only use the coupling length to estimate the crosstalk. In this paper, we present a post-layout timing driven crosstalk optimizer based on the relative window method. According to the relative signal arrival time and the coupling length, we define a delay degradation graph to describe the crosstalks between nets in a routing solution. Our optimization goal is to maximize the time slack by iteratively improving the delay degradation graph without increasing the channel height. Benchmark data consistently show that our post-layout timing driven crosstalk optimizer can further improve the routing solution obtained by a conventional crosstalk optimizer.