Abstract: Mesh size selectivity of trawl cod-end is received more attentions because the by-catch is much serious in trawl fishery. The mesh size selectivity can be expressed by logistic or Richards equation. There are two kinds of selectivity cures, body length or master selectivity curve, because of the variables used in the selectivity equations. During studying the mesh size selectivity of trawl cod-end, cover net methods and compare fishing methods are usually used. The author reviewed different resolution methods and incorporative conditions aiming at above two methods. And the author advanced it is important to study the action between fish and gear, the factors influencing fish encountering gears, and how to found reasonable mathematical models using for estimating master selectivity curve in the study on gear selectivity. 