In this study, the effects on the cod-end selectivity of two different techniques of covered cod-end method used commonly in the trawl selectivity experiments have been tried to determine with towing of 30 and 60 minutes. However, it has been intended the fact that also a possible masking effect of covered cod-end method is determined on the cod-end meshes. Standard and hooped cover cod-ends were rigged with sequence to the 22 mm nominal mesh size (knot to knot) at two seems modern demersal trawl net. The parameter of L-25 is indicate smaller minimum retained proportion at the each length group. Selectivity curves and parameters that are,valid belongs to different covered cod-end techniques have been obtained from red mullet (Mullus barbatus Lin. 1758) had a highest proportion in the catch composition of demersal trawl fishery. The values of L-25 for red mullet has been determined as 10.55 cm and 10.44 cm in the standard covered cod-end technique while the same values were determined as 12.86 cm and 12.13 cm in the hooped cover technique at he towing time of 30 and 60 minutes with respectively. Selection factors obtained approximately 3.0 value. During the study period, the masking effect of covered cod-end which is one of the most important factors effected to the selectivity had been removed relatively by using improved hooped cover technique and it has been obtained more reliable data in order to determine the true selectivity parameters of cod-end.