The multivariate calibration method, partial least square regression (PLS) was applied for the simultaneous spectrophotometry determination of amitriptyline hydrochloride (AMT) and chlordiazepoxide (CHL) in their mixtures. The parameters of chemometric technique were optimized and the proposed method was validated with synthetic samples and applied to analyze these drugs in pharmaceutical products with good accuracy and precision. The results were compared with those given by United State Pharmacopoeia method. The square of the correlation coefficients (R2) for predicted AMT and CHL with the proposed method in a test sample were 0.9951 and 0.9897 respectively. The relative standard deviation for commercial tablets in the proposed method and USP standard method were 0.42 and 2.61%, respectively.