We estimated the transit time of GaInAs/AlAs double-barrier resonant tunneling diodes (RTDs) oscillating at 0.6–1 THz. The RTDs have graded emitter structures and thin barriers, and are integrated with planar slot antennas for the oscillation. The transit time across the collector depletion region was estimated from measured results of the dependence of oscillation frequency on RTD mesa area. The estimated transit time was slightly reduced with the introduction of the graded emitter, probably due to reduction of the electron transition between Γ and L bands resulted from the low electric field in the collector depletion region.