This paper, the first of a series of papers, examines equilibrium properties of N2O4--> <-- NO2 in liquid state by classical molecular dynamics simulations of liquid NO2. An ab initio MO calculation has been carried out to elucidate NO2-NO2 potential, and an orientation-sensitive pairwise potential (OSPP), which can reproduce highly anisotropic character of covalent bonding between N-N, has been formulated. The OSPP potential is parameterized by the well depth D-e and by two anisotropy factors: A(theta) (0 less than or equal toA(theta)less than or equal to1) the anisotropy factor for the rocking angle between NN bond and ONO direction, and A(tau) (0 less than or equal toA(tau)less than or equal to1) for torsional angle of the two NO2 about NN bond. The reactive liquid N2O4 is modeled as liquid NO2 which interacts with the OSPP potential between N-N atoms and Lennard-Jones potentials between N-O and O-O atoms. Equilibrium properties were found to be very sensitive to the well depth D-e and anisotropy factors of OSPP. The population of more than the NO2 dimer (3-mer, 4-mer,...) is considerable when anisotropy factors of the NN bond are small. On the other hand, the equilibrium liquid N2O4--> <-- 2NO(2) is formed, that is, most NO2 form monomer or dimer and the population of more than 3-mer is very small when A(theta)+A(tau)greater than or equal to0.4-0.5. In simulated liquid NO2/N2O4, concentration of N2O4 is found to increase as D-e increases, A(theta) increases, and A(tau) decreases. The equilibrium constant for the dissociation reaction has been derived by computing the potential of mean force as a function of the N-N distance r(c) (the reaction coordinate). The OSPP potential for D-e=0.12x10(-18) J, A(theta)=0.5 and A(tau)=0.1 is found to reproduce the observed liquid phase equilibrium properties fairly well.