This paper relates to some model experiments in respect to shallow water effects upon turning ability which were carried out at the Ship Research Institute under co-operation of the Sasebo Heavy Industries Co. Turning tests were carried out for two model ships, one of a cargo ship (L/B=7.3 B/d= 2.5C_b =0.7) and the other of a mammoth tanker (L/B= 6.41 B/d=2.61C_b=0.81), at various water depth. These tests show that shallow water effect upon turning ability increases slightly in accordance with decrease of water depth, while the depth is deeper than about 2.5d, however increases remarkably when the depth decreases less than 2.5d. For example, the non-dimensional angular velocity, Ω, for 35° helm angle, at water depth of 2. 5 d, is equal to 9095% of the Ω for the same helm angle at deep water, however at 1.3 d, the Ω is equal to 5060% of the Ω at deep water for the same helm angle.