Post filtering is an effective tool for reducing coding artifacts in video coding. Most post-filtering tools are based on lowpass filtering, and an edge preserving filtering is useful because edges have an important effect on visual quality. An edge preserving filtering has an edge detection step, region detection step, and filtering step. The effectiveness of such filtering is much affected by the edge detection feature. Therefore the edge detection threshold should be carefully determined. We created a new method of determining the edge detection threshold of a post filter, that is based on the prefiltering features and encoding conditions. Computer simulations show that this method achieves the best visual quality for given prefiltering and encoding conditions.