A ultrahigh-resolution continuous wave (CW) synthetic aperture radar (SAR) is developed and demonstrated. In the transmitter, photonic frequency-quadruple is used to generate a linearly-frequency-modulated (LFM) radar signal; while in the receiver, photonic dechirp processing is employed to receive the reflection signal. The presented LFM-CW SAR operates in Ku band with a bandwidth of 5000 MHz and is evaluated through a series of imaging tests. The results show photonic technologies are able to handle of transmission and reception of very large bandwidth for ultrahigh-resolution imaging radar.