Abstract: The physical-mechanical properties of transversely isotropic layered slates may be degraded due to water-rock interaction (WRI) and natural weathering, which potentially leads to the instability or collapse of tunnels, slopes and mining. In this study, based on the specimens sampled from the carbonaceous slates of Muzhailing Tunnel, the corresponding drying samples (drying in oven after fabrication) and weathering samples (for 60 days’ natural weathering after fabrication) are prepared, respectively. The Brazilian tensile tests are conducted. The test results show that the two kinds of carbonaceous slates are both characterized by brittle failure, but the mechanical response covering failure displacement and the peak load are rather different. Further, the tensile strength are significantly affected by bedding while the cleavage failure patterns are also dominated by bedding. The softening coefficient of the static weathering samples is 0.11~0.13, which implies that the WRI and natural weathering play a vital role in the course of rock failure, but they have few influences on the transverse isotropic tensile properties of bedding. Moreover, the mechanisms of specimen failure are explained by the SEM technique to analyze the micro-components and observe the process of specimen deterioration due to physico-chemical reaction.