The rotationally resolved high-resolution fluorescence excitation spectrum of the 0–0 band in the S₁ ← S₀ electronic excitation of thioanisole was observed using the techniques of a collimated supersonic jet and a single-mode ultraviolet laser for the first time. High accurate rotational constants for the S₀ and the S₁ states have been determined by precisely calibrated transition energies of about 1000 assigned rotational lines. The molecular structure of thioanisole has been estimated by high-level MO calculations. The planarity of thioanisole in the S₀ and the S₁ states was also demonstrated clearly. The lifetime of the S₁ state was estimated to be 2.0 ns from the observed line width. This line shape did not change with the magnetic field of 1 Tesla, suggesting that the main radiationless process should be internal conversion to the S₀ state.