The accidental release is one of the main risks of carbon capture and storage (CCS) and supercritical carbon dioxide (S-CO2) power cycle system. The release of mass CO2 will present severe risks to pipeline, vessel and surrounding population. The study of CO2 release characteristics is helpful to understand the crack propagation and the diffusion behavior outside the crack. In this paper, to investigate release characteristics in vessel and diffusion phenomenon outside rupture, high pressure CO2 release experiments from pressurized vessel were carried out for different initial states (liquid and gaseous phase), temperature (20.0 degrees C to 50.0 degrees C) and rupture sizes (1.0 mm, 2.0 mm, 3.0 mm). The transient pressure, mass flow rate, wall temperature, external jet structure and phase transition were studied following vessel release. The results show that different initial states will undergo different decompression process. As initial state is in liquid phase, fluid temperature is lower and circumferential wall temperature shows large gradient. Effects of initial temperature and nozzle sizes under different initial states on CO2 release characteristics were obtained. The experimental results show lower temperature will not damage material in experiment and results are of great significance for early leakage detection and model development. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.