Axitinib is a potent second-line molecular-targeted agent for metastatic renal cell carcinoma (mRCC). Axitinib pharmacokinetics and its relation with genetic polymorphisms were evaluated to predict the adverse events (AEs) and efficacy of axitinib. We analyzed 46 patients with mRCC who were treated with axitinib. The plasma axitinib level was measured at 0, 2, 4, 8, and 12 h after administration (C0, C2, C4, C8, and C12; ng/mL) on day 7 of the treatment. Genetic polymorphisms related to axitinib pharmacokinetics, including SLCO1B1, SLCO1B3, SLCO2B1, ABCB1, ABCG2, CYP2C19, CYP3A5, and UGT1A1, were analyzed. Axitinib C0 and AUC0-12 in patients with UGT1A1 poor metabolisers (*6/*6, *6/*28, and *28/*28; n = 10) were significantly higher than those in patients with UGT1A1 extensive metabolisers (*1/*1, *1/*6,*1/*28, and *27/*28; n = 36) (23.6 vs. 7.8 ng/mL, p = 0.030, and 441.3 vs. 217.1 ng h/mL, p = 0.007). The cutoff levels of C0 to predict ≥ G2 hypothyroidism and ≥ G2 anorexia were 6.6 and 7.1 ng/mL, respectively (p = 0.005 and p = 0.035). The overall survival (OS) in patients with C0 > 5 ng/mL was significantly better than that in patients with C0 < 5 ng/mL (p = 0.022). Genetic polymorphisms in UGT1A1 were significantly associated with the plasma axitinib level. The plasma axitinib level was significantly associated with the frequency of AEs and OS in patients with mRCC. No direct relationship was observed between UGT1A1 genotypes and the frequency of AEs or OS.