Layered MAX phase ternary compounds (M = early transition metals, A = group A elements, and X = C or N) show promise of wide applications in many applied fields because these compounds have combined ceramic and metallic properties. As an exemple of the MAX phase family, Ti3SiC2 exhibits a high melting temperature, high electrical and thermal conductivities, and an excellent resistance to oxidation and thermal shock. Particularly, it possesses unusual mechanical properties, such as easy machinability, high Young's modulus, thus it is considered as a candidate in advanced nuclear reactors.In this work, we investigate the effect of hydrogen and helium on the cleavage fracture of Ti3SiC2 in order to evaluate the reliability of Ti3SiC2 used in nuclear industry. We have performed first-principles mechanical calculations by using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Uniaxial tensile simulations along c-axis have been done to calculate the stress-strain curve and the cleavage energy for each interlayer of Ti3SiC2. It is found that Ti3SiC2 has the cleavage characteristics, and the habit cleavage plane starts from Si-Ti interlayer because of relatively weak Si-Ti bond. Hydrogen and helium always accumulate in the Si layer. Helium decreases largely the critical stress of cleavage fracture of Ti3SiC2. In contrast, hydrogen does not efficiently affect the cleavage fracture in Ti3SiC2. The difference between helium and hydrogen behaviors in Ti3SiC2 originates primarily from the difference of electronic hybridization with lattice atoms of Ti3SiC2. For helium, the neighboring Si atoms will be ejected by helium atoms, and the Si-Ti bonds will be broken, thus resulting in the cleavage fracture. However, for hydrogen, it is primarily hybridized with the s states of neighboring Si atoms, which does not severely disturb the p-d hybridization between Si and Ti atoms. Thus, the cleavage fracture from Si-Ti interlayer is hardly aggravated in the presence of hydrogen. Fortunately, Ti3SiC2 has a self-repair ability at high temperatures. It will desorb helium atoms at high helium pressure through Si layers. This behavior will alleviate the cleavage fracture induced by helium. In summary, Ti3SiC2 may be a potential material applied in light water or other fission reactors in the future.