In the present study performance of air-cooled cross flow microchannel heat exchanger is analysed experimentally with various metal oxide type nanofluids. Types of the nanoparticles and their particle size are TiO2 (30 nm, 50 nm), MgO (20 nm, 40 nm), ZnO (10-30 nm, 35-45 nm, 80-200 nm), SiO2 (20-30 nm, 60-70 nm), NiO (10-20 nm) and Fe3O4 (15-20 nm). Zeta potential measurement is employed for the stability evaluation of the dispersions. Among the nanofluids considered, only suspensions containing SiO2 (60-70 nm) and NiO (10-20 nm) nanoparticles give zeta potential above the critical stability value of 30 mV. Strong dependency between relative zeta potential of the suspensions and particle size is observed. The heat transfer characteristic of nanofluids through the cross flow air cooled microchannel heat exchanger is evaluated with the overall heat transfer coefficient under laminar conditions. Experimental results show that, all nanofluids considered, except TiO2/water, deteriorate the heat transfer coefficient of nanofluids depending on the flow rate through heat exchanger. This may be may be explained by the poor nanofluid stability and deposition of nanoparticles to the surface of the channels during the flowing of the nanofluids.