This study aims to examine the mechanical behavior of disintegrated carbonaceous mudstone modified with nanosilica and cement (DCMNC). Many DCMNC specimens with various nanosilica contents were prepared. The X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) observations were performed on some of the specimens. Afterwards, triaxial tests were carried out on the remaining specimens to determine the mechanical behavior of DCMNC. The results showed that the cohesion exhibited a positive correlation with nanosilica content while the angle of internal friction presented a negative correlation with nanosilica content. The peak deviatoric stress, residual deviatoric stress and brittle modulus of DCMNC showed an increase followed by a decrease as nanosilica content varied from 0 to 8%, and all of them reached corresponding maximums at a nanosilica content of 2%. Thus, 2% was considered to be the optimum nanosilica content. The modification mechanism of DCMNC could be explained by the pozzolanic reaction related to nanosilica and the filling effect of nanosilica.