A simple, precise and accurate method to determine ampicillin and dicloxacillin in a pharmaceutical preparation capsule dosage form were developed and validated using liquid chromatography (LC). The LC separation was achieved on an ACE 150 mm x 4.6 mm, 5 µ in the isocratic mode using buffer: methanol (40: 60, v/v), as the mobile phase at a flow rate of 1.0 mL/min. The methods were performed at 220 nm. In LC method, quantification was achieved with PDA detection over the concentration range of 50 to150 µg/mL. The methods were validated and the results were compared statistically. They were found to be simple, accurate, precise and specific. The methods were successfully applied for the determination of ampicillin and dicloxacillin in a pharmaceutical preparation capsule dosage form without any interference from common excipients. The proposed method was validated in terms of precision, robustness, recovery, LOD and LOQ. All the validation parameters were within the acceptance range.