The aim of the present study was to develop a controlled porosity osmotic tablet of Losartan potassium and to evaluate the in vitro release of the drug from the system. The osmotic tablet is developed such that it delivers 8 mg of Losartan potassium over a period of 24 hours. Drug – Excipient compatibility study was carried out using FTIR study. The results showed that there was no interaction between them. Calibration curves of Losartan potassium were constructed in three different pH; Acid buffer pH 1.2, Acetate buffer pH 4.5 and phosphate buffer pH 6.8. Wet granulation produced excellent flow and the granules were compressed on 9/32 concave punches into tablets. The tablets were then coated with a controlled porosity semipermeable membrane of CA with sorbitol as pore former. The post compression parameters namely uniformity of weight, thickness, diameter, hardness, drug content and uniformity of content were evaluated for the coated and uncoated tablets and were found to be within limits. To describe the mechanism of drug release, the optimized formulation was fitted to various models. The drug release was found to follow zero order and Hixson Crowell release. The accelerated stability testing was carried out for 3 months and showed no change in the appearance, hardness, diameter, thickness, friability, drug content and in vitro release. Key words: Formulation, Evaluation, Losartan potassium, Osmatic controlled, matrix tablets