Parecoxib (PX), a prodrug of valdecoxib (VX), is an injectable selective COX-2 inhibitor, and is recommended for the treatment of cancer pain. PX can be rapidly hydrolyzed into its active metabolite VX, and VX is further metabolized into hydroxylated valdecoxib (OH-VX) by cytochrome P450 enzymes. However, cancer patients have been reported to possess reduced drug metabolism ability, which might cause excessive drug accumulation. Such overdose of PX significantly increased the risk of renal safety and cardiovascular events. Therefore, it is necessary to elucidate the concentration profiles of PX and its metabolites in cancer status. In this study, a sensitive, rapid and specific LC–MS/MS method for quantification of PX, VX and OH-VX in the plasma of tumor bearing mouse was developed and validated. After protein precipitation, all the analytes were separated on an Agilent ZORBAX Extend-C18 HPLC column (2.1×100mm, 3.5μm) with gradient elution. The analytes were detected by an electrospray negative ionization mass spectrometry in the multiple reaction monitoring mode. The transition m/z 369.0→119.0, m/z 312.9→117.9, m/z 329.0→196.0, and m/z 307.1→161.3 were used for monitoring PX, VX, OH-VX and IS respectively. The calibration curves of the analytes showed good linearity over the concentration range of 3–3000ng/mL for PX and VX, and 3–1000ng/mL for OH-VX. Intra- and inter-batch accuracies (in terms of relative error, RE<9.9%) and precisions (in terms of relative standard deviation, RSD<8.8%) satisfied the standard of validation. The matrix effect, recovery and stability were also within acceptable criteria. The method was successfully applied to the pharmacokinetics study of PX in tumor bearing mice, and PX and VX levels were found elevated with the growth of tumor volume, which might increase the risk of drug overdose.