Ellipsoidal head is a common end closure of pressure vessel. Plastic collapse is a crucial failure mode considered in the design of ellipsoidal head subjected to internal pressure. Internally pressurized ellipsoidal head tends to be hemisphere (geometric strengthening) due to the effect of material hardening before plastic collapse occurs, which enhances load carrying capacity of ellipsoidal head. However, in the current pressure vessel codes such as ASME BPVC.VIII.1 and BPVC.VIII.2, EN 13445-3, and Chinese codes GB/T 150.3 and JB 4732, design methods based on linear elastic or perfectly-plastic theory are used to prevent plastic collapse of ellipsoidal head, leading to conservative design. Therefore, we developed a new method for preventing plastic collapse of ellipsoidal head under internal pressure, considering the effects of material hardening and geometric strengthening. The new method was developed on basis of our previous extensive work on finite element analysis and experiments for plastic collapse of internally pressurized ellipsoidal heads. The new method provides sufficient margin of safety by checking against the experimental bursting results of full-scale ellipsoidal heads with various geometries, various material types and various manufacturing methods. Compared with the design methods in the current pressure vessel codes, the new method shows an advantage of economy. This new method had been approved by China Standardization Committee on Boilers and Pressure Vessels, and at present it has been introduced into the Chinese pressure vessel code.