Spin-one Ising model with biquadratic exchange interaction is investigat ed within functional integration approach. Free energy in the approximation followi ng random phase approximation (RPA) is calculated. Expression for ferromagnetic ordering temperature in RPA is obtained. Phase diagram of the system is studied by means of the free energy expansion. System's properties dependence on the ratio of bilinear to biquadratic exchange interaction constants is discus sed. Structurally disordered model is considered. Ferromagnetic transition temperatur e change caused by structural disorder is found. Results of numerical simulations for hard core system's structural factors are presented.