The interaction of losartan potassium (LP) with bov ine serum albumin (BSA) was studied by fluorescence quenching in combination with UV–Vis spectroscopic method und er near physiological conditions. The fluorescence quenching rate constants and binding constants for BSA–LP sys tem were determined at different temperatures. The fluorescence quenching of BSA by LP is due to stati c quenching and energy transfer. The results of the rmodynamic parameters, ∆ H ( ‒ 134.3 kJ mol ‒ 1 ), ∆ S ( ‒ 368 J mol ‒ 1 K ‒ 1 ) and ∆ G ( ‒ 24.52 to ‒ 20.83 kJ mol ‒ 1 ), indicated that van der Waals interaction and hydrogen bonding played a maj or role for LP–BSA association. The competitive exp eriments demonstrated that the primary binding site of LP on BSA was located at site II in sub-domain IIIA of B SA. The distance between LP and a tryptophane unit was esti mated to be 3.183 nm based on the Förster resonance energy transfer theory. The binding constant (K a ) of BSA–LP at 298K was 1.932×10 4 L mol ‒ 1 . Synchronous fluorescence and three-dimensional fluorescence studies showed t hat the presence of LP could change the conformatio n of BSA during the binding process.