Background: Phyllanthus urinaria L. showed significant hepatoprotective and anti-HBV activities. A sensitive and rapid HPLC–MS/MS method has been developed and validated for the simultaneous determination of hypophyllanthin, phyllanthin, nirtetralin and niranthin from Phyllanthus urinaria L. in rat plasma. The analytes were extracted from rat plasma with n-hexane/isopropanol and diazepam was used as the internal standard. Results: The calibration curves were linear (r > 0.9971) over a concentration range of 2–1000 ng/ml for hypophyllanthin and nirtetralin, and 1–1000 ng/ml for phyllanthin and niranthin. The precision, accuracy, recovery and stability were deemed acceptable. Conclusion: The HPLC–MS/MS method was successfully applied to a PK study of the four lignans in rats after oral administration of Phyllanthus urinaria L. extract.