Direct numerical simulation (DNS) of transcritical heated turbulent boundary layers on a zero-pressuregradient flat plate at supercritical pressures is conducted by solving the Ml 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 reported. The results indicate the importance of the compressibility effects (especially contribution of the pressure-dilatation) in the transcritical turbulent boundary layers even in a low Mach number condition. The analyses suggest that the compressibility effects originate from the abrupt variation in the density in the transcritical regime, which induce the significant expansion processes with a small temperature change through Tpc and causes the peculiar reduction of the Reynolds stresses at the transcritical condition.