Course stability of towed large barge has recently become very important because of the increase of the cargo transportation by “Barge System” in harbor, cannel and coastal sea which intends the high work efficiency and the decrease of man's work in the collection and delivery of cargoes. Most of existing barges have, in general, large B / d and small L / B ratio that may cause a poor stability on course. Therefore, these barges, which are often installed a pair of skegs to improve the course quality, are towed not to cause yawing motion by the bridle or two lines towing for the safe and quick towing operation. Strandhagen et al., Terao, Inoue, and Bindel reported the worthy theoretical works on this essential problem. These papers offer much knowledge on stability criterion of towed ship, but towing test to confirm their theories would not be reported as far as we know. In this paper, authors discuss the following several points to clarify the validity of the above mentioned theories. The first point is the theoretical and experimental study on the quantitative difference in derivatives due to skeg section shape. Applying wing theory to skeg, the authors modified the coefficients of Strandhagen's equation of motion, and calculated the additional term of derivative, L _β' & L _γ' (corresponding to lift coefficient in wing theory), using the results of forced yawing test (P. M. M.). These values were compared with the theoretical calculation by three dimensional wing theory and obtained an agreement in good order. The next subject is to know the effect of skeg shapes and tow line length on course stability in towing condition. The Routh-Hurwitz's criterion was used to select the best skeg section shape and its attachment angle. In the last step, heading angle or turning rate was simulated by “Random Search Method”. The simulated time histories showed a good agreement with the measured phenomena at towing test. As a result of this work, it is verified that the theory by Strandhagen (including our modified equation) gives a good estimation of course stability of towed ship for practical usage.