Nanofluids, because of their attractive properties, innurmous benefits and application in many fields such as micro electronics, traort and biomedical attract researchers have focused on high conduction heat transfer coefficient of nanofluids but have done less efforts to apply nanofluids as a new generation of coolants. In present study, boiling of water-silica nanofluids on circular flat plate heater, to observing the effect of inclination angle of heater surface on CHF and BHTC was done. The experiments were performed by focusing on surface morphology. The nanofluid concentration was between 0.001-0.01 vol. % and inclination of heater was varied from 0? to 90?. The results showed CHF increases with nanofluid concentration.The CHF enhancement in horizontal configuration for 0.001, 0.0025 and 0.005 vol.% was 25,53 and 65%, repectively. It was observed BHTC increases with nanofluid concentration in the range of 0.001-0.005 vol. % but when concentration increase to 0.01 vol. %, BHTC decreases. The BHTC enhancement in horizontal configuration for 0.001, 0.0025,0.005 and 0.01 vol.% was between 10-20, 62-166, 75-177 and 40-153%, respectively. Discovering the effect of inclination angle of heater surface on boiling on DI water and nanofluid showed with increasing heater orientation from 0? to 90?, bubbles moved along the heater surface and departure of bubbles from heater surface become harder. This occurrence decreases BHTC. In boiling of DI water, CHF decreases by inclination angle of heater surface. Insomuch, the bubbles sliding on heater surface and coalescence together, vapor film formed on heater surface in less heat fluxes and CHF decreased. But inclination angle of heater surface had a different effect on CHF of nanofluid so much that CHF increased with inclination angle of heater surface. It showed that the effect of nanoparticle deposition on heater surface dominates on bubble movement. The Atomic Force Microscopic (AFM) analysis of heater surface before and after boiling of nanofluid over horizontal and vertical heater surface showed in vertical configuration more uniform nanolayer forms on heater surface that increase CHF. Keywords: pool boiling, nanofluid, flat plate, inclined surface, critical heat flux, pool boiling heat transfer coefficient