In the broader views, all fishing gears are environmentally damaging toa greater or lesser degree. Many fishing gears are unselective gears in thatthey catch a wide range of individual size and a much larger number of differentspecies. Most of the gear development technology is aimed to reduce the catch ofsmall individuals and non-target (by-catch) species. Gillnets in Songkhla Lake,Thailand are used various mesh sizes to catch the same species of mullet (~iLisasubviridis~i) e.g. 3.8, 4.0, 4.2, 4.5 and 4.7 cm. Consequently, a large part ofspecies was almost be caught before they could spawn at the first time, so thefish do not have a chance to spawn and reproduce. This condition willdeteriorate fish species because this species can not reproduce and graduallythe species will be extincted. Therefore, there is a vital need to study aboutan optimum mesh size of gillnet to catch mullet in a sustainable manner. The length frequency distribution of mullet collected from gillnet withdifferent mesh sizes (4.0 cm and 4.7 cm) had overlapping mean length values at21.1, 23.5 and 25.2 cm. Based on the calculation and the curves from lengthfrequency distribution of mullet occurred at point of length (L(,0)) = 23.52 cm.With guided by level mature of gonad and coefficient value of mullet (k) =0.1838, hence obtained the optimum mesh size of gillnet was 4.3 cm as for the fishery point of view. The optimum mesh size of gillnet as a biological point ofview with entering the size of length (L(,m) 50%, for male and female fish were20.1cm and 23.1 cm, respectively) at equation length and girth of mullet, henceobtained the girth size for male and female fish were 102.5 mm and 118.2 mm,respectively. Enter those values at equation: Mg = 0.44 x G, hence obtained theoptimum mesh size of gillnet was 4.5 cm .for male and female was 5.2 cm.Therefore, gillnet with mesh size 4.0 cm can not be use as a, mesh size to catchmullet. The marketable size of mullets should not less than 23.5 cm.