Background & Aims: Losartan is a non-peptide potent antihypertensive agent that acts through blocking angiotensin II receptors. Hyzaar® is a combination product that contains two drugs, losartan and hydrochlorothiazide, used to lower high blood pressure. There are some reports regarding simultaneous measurement of the drugs in pharmaceutical and biological samples which includes HPLC, CE, CEC, and multisyringe chromatography. Methods: UV-V is spectra of standard solutions of losaratan and hydrochlorothiazide were prepared separately and together in combination with various concentrations of the drugs. To determine these two drugs simultaneously without any preliminary treatment, losartan was determined by ratio derivative spectrophotometry at third derivation with  = 10 nm at 246.3 nm, and hydrochlorothiazide was determined at first derivation with ∆ = 5nm at 334.4 nm. This method was used to determine the two drugs in real samples of tablets. Results: The method had a good linearity in the concentration range studied (r > 0.999). Precision of the method revealed that RSD% was lower than 2.10 and 1.79 for losartan and hydrochlorothiazide, respectively. Accuracy of the method on the basis of error% was lower than 3.3% for losartan and 2.3% for hydrochlorothiazide. Based on the validation results, it could be concluded that the method was reliable and valid for determination of the drugs in their preparations. Real sample analysis showed that tablets had 84.9% hydrochlorothiazide and 92.6% losartan compared to label amount of the drugs. Conclusions: Results depicted a simple, valid, inexpensive and reliable method for simultaneous determination of the two drugs in pharmaceutical preparations applicable to the quality control laboratory of pharmaceutical industries.