In this paper, we analyze the criterion of squint angles for repeat path InSAR requirement. The squint effect is produced by the non-orthogonality between the antenna beam and SAR platform flight track. For SAR image processing, Doppler centroid estimation is essential for correcting the squint effect. For repeat path InSAR application, squint difference between two flight paths degrades the InSAR coherence of 2 images. InSAR processing generally requires the estimates of the parallel and perpendicular baselines. Depending on the wavelength, critical baselines must be met. To improve the baseline selection, a new index Q, based on a differential squint angle, is introduced in this paper. A time-delay active radar calibrator(TD-ARC) was used to evaluate the squint angles over real SAR images. Comparing the squint angle with refined Doppler centroid estimation, the accuracy is less than 0.03degrees. Finally, we use over 36 SAR pairs from ERS-2 to verify index Q as an InSAR pair selector. Results indicate the Q index is very effective in selection of InSAr pair with almost 100％ successful rate.