Two-boat trawl is generally in use for fishing prawn (Penaeus orientalis) at Po Haiarea in autumn. Po Hai is not only the main fishing ground for prawns, but is also thespawning and nursery ground of many commercial fishes. Unfortunately, a considerablenumber of young fishes have been destroyed in the prawn trawing every autumn. It is avery serious problem for the fish stock, so that, a new two-boat long square prawn trawlwas designed in 1974--1977, in order to increase catching efficiency and to reduce thedamage on young fishes. The characterstics of this newly designed trawl are: its square was twice as largeas that of the original one, the side panels were added to increase the covered net moutharea and the height of trawl, in order to prevent prawn in jumping over the headline ofthe net when they touc on the ground rope, and its cod-end mesh size was increased to50 mm. Consequently it saves a large quantity of young fishes from escape. The experimental results are satisfactory. In comparing with the old trawl model inwater tank, the principal advanges of new trawl model are the lower resistance, the higherheadline and the larger area of the covered mouth. The fishing operations have provedthat the eatching efficiency is increased over 40% and the damage to young fishes redu-ced over 60%.