The conventional Wavenumber domain algorithm(WDA) employs Fast Fourier transform(FFT) to implement matched filter. It is characterized by its high precision SAR imaging. Compared with FFT, Fractional Fourier transform(Fr FT) can provide lower and narrower sidelobes when it is applied to compress chirp signals at the optimal transform. This paper proposed a novel SAR imaging algorithm, termed Fr WDA which replaces FFT in WDA with Fr FT. The Fr WDA was derived and its implementation was presented. How to choose the optimal transformation order was given subsequently. The Fr WDA preserves high precision focusing, and has better performance on sidelobe suppression. The Fr WDA is more suitable for the high precision SAR imaging, especially in target detection and recognition. Simulation results demonstrate that Fr WDA provides better performance on focusing and sidelobe suppression compared with the classical FFT-based WDA. Compared with FFT, Fractional Fourier transform (Fr FT) can provide lower and narrower sidelobes when it is Applied to compress chirp signals at the optimal transform. This paper proposes a novel SAR imaging algorithm, termed Fr WDA which replaces FFT in WDA with Fr FT. The Fr WDA was derived and its implementation was presented. How to choose the optimal transformation order was The results show that Fr WDA provides better performance on focusing. and the better performance on sidelobe suppression. The Fr WDA is more suitable for the high precision SAR imaging, especially in target detection and recognition. and sidelobe suppression compared with the classical FFT-based WDA.