Abstract: A charge transfer complex based on the reaction of roxithromycin and erythrosine B was formed in acetic acid medium, which showed a maximum absorption wavelength of 520 nm. Spectrophotometry was applied to the determination of roxithromycin in roxithromycin capsules. The optimized conditions found were as follows:① 4.0×10^-4mol·L^-1 acetic acid solution was used as reaction medium; ② temperature of reaction was 25℃; ③ time of reaction was 20 min; ④ amount of 2.0×10^-4mol·L^-1 erythrosine B solution was 1.2 mL. Linear relationship between values of absorbance difference of the reaction system and mass concentration of roxithromycin was obtained in the range of 0.020-0.500 mg·L^-1, with detection limit (3s/k) of 0.014 mg·L^-1. The proposed method was applied to the analysis of samples of roxithromycin capsules, giving values of recovery found by standard addition method and RSDs (n=5) of determined values in the ranges of 101%-107% and 0.84%-1.6% respectively.