The power gap around 0.5 Omega(e) (where Omega(e) is the equatorial electron gyrofrequency) of whistler-mode waves is commonly observed in the Earth's inner magnetosphere, but its generation mechanism is still under debate. By performing two-dimensional particle-in-cell simulations in a uniform background magnetic field, we investigate the spectral properties of whistler-mode waves excited by temperature anisotropic electrons. The waves have positive growth rates in a wide range of normal angles (theta approximate to 0 degrees-35 degrees),...