In this study, by-catch reduction in trammel nets used for prawn fishery in Izmir Bay Was Studied between May and October 2003. In shrimp trammel net fishery catching of discard species is an important problem due to a series of reasons such as disentangled species causes extra work for fishermen and decreases the labour life of nets. A Guarding net was attached to lead line of traditional shrimp trammel net as a modification to minimize this problem. Control Nets (CNet) originated from traditional commercial nets and were compared against two groups of Experimental nets (Exp1 and Exp2) in two periods. In the first period of the stud, Exp1 net group caught 36.46% fewer shrimp, (Melicertus kerathurus) in comparison to CNet. In addition to this, 35.62% crab (Goneplax rhomboides), 40.71% mantis shrimp (Squila mantis) and 44.77% purple dye murex (Bolinus brandaris) were caught fewer than those in CNet net group. In the second period of the study, Exp2 net group was put into the trial. In this period, Exp1 net group compared with CNet net group caught less shrimp (8.09%), crab (66.15%), mantis shrimp (26.79%) and purple dye murex (32.25%). Exp2 net group caught fewer shrimp (0.99%), crab (50.63%), mantis shrimp (17.33%) and purple dye murex (25.92%) than CNet net group. This study indicates that the net modification was capable of reducing bycatch. Rising of trammel net above the lead line from muddy ground prevents discarding of demersal species from net. Therefore, the height of guarding net is an important criterion for bycatch reduction.