Abstract: The material of heavy-load gear,18CrNiMo7-6,was carburized after manufacture. Gears could not be tested exhaustively because the physicochemical parameters of surface and sub-surface material were measured by destructive methods. This paper presented a nondestructive method based on Barkhausen effect. The common six features of MBN (Magnetic Barkhausen Noise) signal were extracted for PCA (Principal Component Analysis). The principal components from dimension reduction process were used to evaluate the martensite, residual austenite and case depth in the surface and sub-surface of carburized 18CrNiMo7-6. By evaluating some products from gear faculty, this nondestructive testing and evaluation method was proven effective for gear exhaustive test.