Quadratic phase error (QPE) of SAR (Synthetic aperture radar) echo accumulated during SAR integration interval, determines SAR image resolution. QPEdepends on quadratic position error of SAR antenna.Though INS (Inertial navigation system)/GPS (Globe position system) integrated navigation system is widely usedin SAR motion compensation, in each coherent time, however only INS data is used for the measurement of SARantenna position and GPS data is avoided due to its highfrequency errors. So INS errors play main role in the SARimage resolution. There are many sources of INS errors,such as gyro drift, accelerometer bias etc. In order tofind the analytical relation between INS errors and QPEof SAR we have written their transfer functions in Laplacedomain. These transfer functions depict the different effects of different INS errors on QPE of SAR. Time domainsimulations and SAR imaging simulations prove the analytical conclusions. In this paper the analytical relations havebeen verified by computer simulations. The results in thispaper are useful for developing SAR motion compensationsystem with high performance and cost ratio.