The spin-torque oscillator, which can generate an AC voltage oscillation with the same frequency, have attracted considerable attention due to its potential applications in the frequency-tunable transmitters and receivers for wireless communication and the recording heads of high-density hard disk drives. However, from the energy-balance equation's point of view, in the absence of in-plane shape anisotropy of spin torque oscillator, the energy supplied by the spin torque is always larger than the energy dissipation due to the Gilbert damping, thus, a finite magnetic field applied perpendicular to the plane is required for a steady-state precession. This feature has limited its potential applications. In this paper, the influence of the intrinsic in-plane shape anisotropy on the magnetization dynamics of spin torque oscillator consisting of an in-plane polarizer and an out-of-plane free layer is studied numerically in terms of the Landau-Lifshitz-Gilbert-Slonczewski equation. It is demonstrated that the additional in-plane shape anisotropy plays a significant role in the energy balance between the energy accumulation due to the spin torque and the energy dissipation due to Gilbert damping, which can stabilize a steady-state precession. Therefore, a stable self-oscillation in the absence of the applied magnetic field can be excited by introducing additional in-plane shape anisotropy. In particular, a relatively large current region with zero-field self-oscillation, in which the corresponding microwave frequency is increased while the threshold current still maintains an almost constant value, can be obtained by introducing a relatively large intrinsic in-plane shape anisotropy. Our results suggest that a tunable spin transfer oscillator without an applied magnetic field can be realized by adjusting the intrinsic in-plane shape anisotropy, and it may be a promising configuration in the future wireless communications.