Casimir vacuum has many peculiar properties, e.g. the speed of light can exceed c. This paper investigated the behavior of gas molecules in Casimir vacuum. Analysis and calculation indicate that we can get a new absolute zero in Casimir vacuum. This is due to the Hamiltonian, which equals the kinetic energy of the ensemble of gas molecules that is reduced in Casimir vacuum; therefore, the gas constant R is changed. This conclusion has great significance for the research of the vacuum properties.