Nanofluids are considered as promising heat transfer fluids due to their enhanced thermal properties. Addition of nanoparticles in transformer oil can increase overall heat transfer performance and life span of transformer oil. Transformer oil is used in oil filled distributor transformers to dissipate heat from the system which operates on the principle of natural convection heat transfer. Numerical studies are carried out in slice model of distributor transformer using carbon nanotubes (CNT) and graphite based nanofluids. Thermal analysis of designed geometry is studied at varying nanoparticle concentrations (0-2 vol) in transformer oil using 3-Dimensional simulations. The slice model is developed with defined heat flux as boundary conditions at winding and core area. The temperature and velocity profiles are investigated at different particle loadings. Overall heat transfer coefficients of transformer oil with and without nanoparticles are calculated and compared. Nusselt and Rayleigh numbers are calculated to understand the natural convection behaviour of nanofluids inside the geometry. It is found that addition of CNT and graphite nanoparticles in transformer oil at varying particle loadings have significant impact on thermal performance of transformer oil due to better heat transfer coefficient than transformer oil. © 2016 The Authors.