The design of Gas-Freeing system for optimum performance requires much more information on the flow velocity and gas concentration distribution occurring between the obstructive baffles and the structural members in a cargo tank during gas-freeing. In the above point of view, tests were carried out to investigate flow velocity and gas concentration change at the various positions in the scale model tanks which correspond to 1/10 th and 1/50 th of wing tank for a existing 150, 000 D/W ton crude oil tanker. At first, flow patterns were observed using transparent and colored water which visualize the flow behaviors in 1/50 th scale model as shown in photo-3. In next, using CO₂ and air, which are to represent crude oil vapor and vent gas respectively, gas-gas system was studied varying systematically vent air flow rate and initially filled gas concentration. The followings were proved : (a) Three paticular flow fields were observed in the tank from vent air inlet to exit as follows : (1) Jet stream zone. (2) Recirculating stream zone, where the fluid flows induced by jet. (3) Slow stream zone not affected by jet. The last zone which is usually widest of three and involves difficulties of gas-freeing was especially focussed in the thesis. (b) In this zone, vent air stream which flows penetratingly through residual gas causes to deteriorate gas-freeing performance. (c) Densimetric FROUDE number Fr' has significant influence on the gas concentration distribution especially in the third flow field and is proved to be most important criterion in similarity of gas-freeing. (d) A few investigations carrid out with actual gas-freeing in large tankers revealed that the temparature difference between air and initially filled gas, which is sometimes affected by tank washing with hot water, also has considerable influence on the performance of gas-freeing.