Calculations in Gaussian approximation and mean-field approximation are performed with the Landau-deGennes model of first order phase transitions. Some "critical exponents" are obtained. For example, in the Gaussian approximation, α = 0.5 ≠ α＇,γ= 1 ≠ γ＇, ν = 0.5 ≠ν＇, etc. Specific heat is calculated and compared with experimental results of liquid crystal MBBA. The nematic-isotropic phase transition is discussed in relation to tricritical points. In this paper, the similarities and differences of the critical properties between first and second order phase transitions are emphasized and discussed. In our opinion, results from scaling laws of second order transitions cannot be simply applied to the case of first order transitions.