Validation of stability indicating RP-UPLC assay method for the determination of Lacosamide and Quetiapine fumarate in pharmaceutical products. This method has showed a high degree performance in separation and quantification of Lacosamide and Quetiapine even in the presence of its impurities. The separation was quite efficient on waters acquity BEH C18 column (50 mm×2.1 mm, 1.7 μm) having used the phosphate buffer pH 2.5 and acetonitrile in the ratio of 75:25 v/v with a flow rate of 0.2 mL/min. The detection was carried out at 210 nm and the elution run time was 5.0 min. The method was linear and found to be in the range between 0.0149 μg/mL-74.71μg/mL (r2=0.9999) and 0.0148 μg/mL-74.67 μg/mL (r2=0.9999) for Lacosamide and Quetiapine respectively. The limit of detection 0.0149μg/mL, 0.0148μg/mL and quantification 0.031 μg/mL, 0.030 μg/mL for Lacosamide and Quetiapine were determined respectively. The precision result of the method ‘% RSD' is less than 0.70 % and the recovery % of Lacosamide, Quetiapine is between 99.1 and 99.9. When the drug is subjected to different stress conditions, the consequential degradation products obtained could not interfere with Lacosamide and Quetiapine during the determination.