The ignition delay times of gas-phase 1,3,5-trimethylbenzene/air mixtures were measured behind reflected shock waves in a heated shock tube facility. The experimental conditions spanned as follows: temperatures of 10801560 K, pressures of 1.020.0 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Ignition delay times were determined using pressure and CH* emission signals monitored at the sidewall. The increasing temperature or pressure results in decreasing ignition delay time definitely. However, the effect of equivalence ratio on the ignition delay time is complex. Lean mixtures are fastest to ignite at temperatures above 1390 K at 1.0 atm, whereas they are slowest to ignite below 1300 K at 20.0 atm, and the equivalence ratio does not have an evident effect on the ignition time at 5.0 atm. The global activation energy decreases dramatically as the pressure increases at all equivalence ratios. Current results are in good agreement with other shock-tube experimental data, and comparisons of current data to kinetic predictions from two recently published mechanisms are presented. In addition, sensitivity analysis was performed to access the important reactions in the ignition process. To our knowledge, this study provides the first ignition delay time data for 1,3,5-trimethylbenzene at a pressure of 1.0 atm. Present results extend the database for 1,3,5-trimethylbenzene ignition delay time.