Abstract: Nanofluids are suspensions of nanoparticles in base fluids, a new challenge for thermal sciences provided by nanotechnology. Nanofluids have unique features different from conventional solid-liquid mixtures in which mm or μm sized particles of metals and non-metals are dispersed. At this research, adding of small weight percent of magnesium oxide (MgO) (0.1, 0.2, 0.4 and 0.5 wt.%) to pure water as working fluid in the building heating system by solar energy gave us different behavior, adding 0.1, 0.2 wt.%, gave low temperature gradient than the pure water while 0.4 and 0.5 wt.%, gave greater temperature gradient than pure water which is desired.