The definitions of the temperature in the nonextensive statistical thermodynamics based on Tsallis entropy are analyzed. A definition of pressure is proposed for nonadditive systems by using a nonadditive effective volume. The thermodynamics of nonadditive photon gas is discussed on this basis. We show that the Stefan-Boltzmann law can be preserved within nonextensive thermodynamics. PACS : 05.20.-y, 05.70.-a, 02.50.-r 1 Tsallis entropy is given by S = i p q i −1 1−q , (q ∈ R)[1]