A lack of ductility is one of the major problems for well cement durability especially under frequent thermal shock conditions typical for geothermal wells. Use of polymeric fibers to improve cement performance is limited under conditions of elevated temperatures. Nanomaterials may provide an alternative solution for improved cements performance due to their unique properties. In this work we tested effect of several types of carbon nanoparticles and nano montmorillonite clay (nanoclay) on short-term mechanical properties of high-temperature cementitious composites after a short hydrothermal curing at 150oC and 250oC. Changes in cement hydration were monitored with an isothermal calorimeter, phase composition and morphological features of the tested cements were analyzed using XRD and Scanning-Electron-Microscope (SEM) coupled with Energy Dispersive X-ray (EDX) techniques. The study showed that these nanomaterials can successfully improve mechanical properties of tested composites. The effective concentrations were higher for nanoclay than for graphene particles (~3%bwob vs. <0.01%bwob respectively). The largest effect of nanoclay on composites cured at 150oC was on compressive strength (strength increase between ~40 (calcium phosphate cement) and 100% (Thermal Shock Resistant Cement, TSRC), while graphene additions to the blend of TSRC cured at 250oC mostly increased its toughness (by up to 300%). Microstructural characterization revealed decreased roughness of nanoparticle modified samples.