An equation of state of the following form P=(RT/V)[+(B(T))/V+(C(T))/(V^2)]+I/(V^9)+α/[(V-β)^2+γ^2] is proposed to describe the P-V-T relation of argon, including its gaseous, liquid and solid phases. The parameters are determined to give good agreements with experimental data, especially in the range between the triple point and the critical point of argon. The transitions and the equilibrium among these phases can thus be expressed through this single equation of state.