A method for the simultaneous determination of aristolochic acids A and B in some Chinese herbals and traditional Chinese patent medicines by ultra high performance liquid chromatography-triple quadrupole mass spectrometry (UHPLC-MS/MS) was developed. Fourteen samples, including Radix Bupleuri, Radix Glycyrrhizae, Radix Platycodonis, Longdanxieganwan, Xiaopangwan, Slimming Tea, etc., were extracted with methanol-water (70: 30, v/v) and purified with Oasis MAX solid-phase extraction cartridges, then analyzed on an Eclipse RP HD C18 column (150 mm x 2.1 mm, 1.8 microm) using 5 mmol/L ammonium acetate solution (pH 7.5)-acetonitrile (75: 25, v/v) as the mobile phase. The mass spectrometric acquisition was carried out by means of electrospray ionization in positive mode (ESI+) with multiple reaction monitoring (MRM) method. The good linearities (r2 > 0.995) were achieved within the ranges of 0.5 - 200 microg/L and 1 - 200 microg/L for aristolochic acids A and B, respectively. The limits of detection (LODs) were 5 microg/kg for aristolochic A and 7.5 microg/kg for aristolochic B, and the limits of quantification (LOQs) were 12.5 microg/kg and 25 microg/kg, respectively. The recoveries of aristolochic acids A and B at the spiked levels of 100 microg/kg and 500 microg/kg ranged from 60.3% to 96.4% with the relative standard deviations (RSDs) not more than 10.2%. The results demonstrated that the proposed method is efficient, sensitive, reproducible, reliable and suitable for the trace determination of aristolochic acids A and B in Chinese herbals and traditional Chinese patent medicines.