In order to obtain the collapse strength data on the spherical shells suitable to the pressure capsule of DSSV, collapsing tests were conducted using spherical shell models by means of MHI 1, 200 kg/cm² hydrostatic tank. These models were made from several kinds of materials including ultra-high yield strength steels such as 18% Ni maraging steel, 10% Ni dual-strengthened steels etc., and machined into near-perfect spherical shape or spheres with initial imperfection of various, thicknesses. From these experiments the following conclusions are obtained : (1) Collapse pressure of relatively thick shells ( h_a / R ₁₀≥0.03) agrees to the theoretical inelastic buckling pressure by Gerard et al. (2) The effect of initial imperfections are evaluated by local radius in case of relatively thick shells. But for thinner shells, this method is not sufficient and nonlinear elasto-plastic analysis will be required. (3) Present results will not always agree with Krenzke's data, especially in thinner shells with flat spot. (4) Fracture appearance of collapsed shells are closely related to the fracture toughness of materials.