The integral hydro-bulge forming (IHBF) process, also called shell hydroforming, is a die-less forming technique used to manufacture hollow parts from preforms made of sheet blanks cut and welded together. In this study, integral hydro-bulging of spherical vessels was investigated both numerically and experimentally. Numerical simulations were performed using Abaqus commercial software, and the numerical results were validated via comparison with those obtained from the experiments. The thickness distribution along the equatorial and meridian paths, sphericity of the formed vessel, and critical fluid pressure at which instability occurs were studied, and the effect of the number of lateral petals was investigated. For this study, an St12 steel sheet with a thickness of 0. 8 mm was used to make a spherical shell with 12 lateral petals and a diameter of 400 mm. The results showed that the use of preforms with more lateral petals leads to a more uniform thickness distribution and shape accuracy in the formation of spherical vessels. Furthermore, by increasing the number of petals from 8 to 16, the thickness decreased by 4. 2%. The maximum thickness reduction occurs in the 8-petal state, and the least thickness reduction occurs in the 12-petal state. The results show that the increase in the number of lateral petals leads to the increase in the critical fluid pressure. By doubling the number of lateral petals, fracture pressure increased by 74% consequently.