This paper deals with the longitudinal stability of the submarine in the deeply submarged condition with controls locked, and presents a method of model experiments of the submarine. Equation of motion of the submarine in the deeply submarged condition is shown in equation (1). Tank tests on models of ellipsoid were conducted to obtain the coefficients and derivatives in this equation. Three components tests were performed on a 4m model and forced oscillation tests on 2m and 4m models, with and without fins. Typical examples of three components tests are given in Figs. 6 and 7 and those of forced oscillation tests in Figs. 8, 9 and 10. Damping coefficients in pitching motion, -Cmq= B /1/2. ρ∇4/S V and added moment of inertia coefficients about centre of buoyancy, K = ( I - I ₁)/ I ₂ are shown in Fig. 12. As an application of the equation of motion (1), stable region of velocity on an ellipsoid with and without fins are shown in Figs. 17 and 18.