Simultaneous determination of serum flecainide and its oxidative metabolites was carried out by using high performance liquid chromatography (HPLC) equipped with conventional octadecylsilyl silica (ODS) column and fluorescence detector. Flecainide and its metabolites, m-O-dealkylated flecainide (MODF) and m-O-dealkylated lactam of flecainide (MODLF) in serum were extracted with ethyl acetate. The recoveries of flecainide, MODF and MODLF were greater than 92, 93, and 60% with the coefficient of variations (CVs) less than 3.2, 5.8, and 5.3%, respectively. The calibration curves were linear at the concentration range of 50-1500 ng/mL for flecainide and 10-500 ng/mL for MODF and MODLF (r > 0.999). The CVs for intra-day assay were 2.7-5.3% for flecainide, 3.0-4.2% for MODF, and 3.7-4.3% for MODLF, respectively. The CVs for inter-day assay were 7.0-8.4% for flecainide, 3.3-6.7% for MODF, and 4.4-7.7% for MODLF, respectively. This assay method can be used for assessing the metabolic ability of flecainide in the patients with tachyarrhythmia.