Direct numerical simulation (DNS) of zero-pressure-gradient heated transcritical turbulent boundary layers on a flat plate at supercritical pressure conditions is conducted by solving the full compressible Navier-Stokes equations. To the best of my knowledge, the present DNS is the first DNS of zero-pressure-gradient flat-plate transcritical turbulent boundary layer. The peculiar interactions between the strongly non-linear real fluid effect at the transcritical condition and wall turbulence, and their resultant turbulence statistics and turbulent kinetic energy budget are investigated. The significant density fluctuations (formula presented) that originate from the strongly non-linear real fluid effects at the transcritical condition play an important role in determining the behaviors of transcritical turbulent boundary layers. Although there are considerable density fluctuation effects on the turbulence statistics, the mean density weighted Van Driest transformation of the mean velocity successfully collapses the mean velocity profiles to the same log-law as seen in ideal gas turbulent boundary layers. We also showed the validity of the semi-local wall coordinate y∗ = y/δ∗v and (formula presented) normalization for TKE budget terms in the transcritical turbulent boundary layers.