Electrohydrodynamic instabilities of a planar nematic liquid crystal (εαα>0) under an inclined electric field (at an angle θ0 with the normal of the glass plates of the cell) are considered. On the basis of the one-dimensional theory of Dubois-Violette et al, conditions for the onset of instabilities under the excitation of static, sinusoidal and square wave electric Fields, respectively, are obtained. Major results include: (1) Relaxation time of charge increases monotonically with θ0. (2) For MBBA, there exists a critical angle θc(ω)(ω is frequency of external field), when θ0> θc(ω) instability cannot occur. θc(0) =54.2°. (3) For MBBA, threshold voltage is a monotonic increasing function of θ0.