Titanium dioxide is important pigment which is used in paint technology. In this project, the physical effect of micro titanium dioxide treated by silica and alumina was studied on rheological, mechanical and corrosion of hydroxylated polyester/ melamine based coating on galvanized steel. This research work was done in three phase: paste preparation and investigation of their dispersion, paint synthesis and evaluation of mechanical and corrosion properties of coatings on galvanized steel. The pastes were prepared in three categories: Micro, Nano and combined (mix of nano and micro). The micro pastes were prepared based on specified formulations and procedure, as follows: 30 min mixing by HSD mixer and 120 min milling by basket mill. Nano paste was prepared based on specified formulation and procedure, as follows: 60 min mixing by HSD, 20 min sonicating by ultrasonic bath, 4 hr milling by basket mill, and 20 min homogenizing by homogenizer. Combined paste was prepared based on a specified formulation and procedure by 1% nano replacement with 4.85% micro, as follows: 60 min mixing by HSD, 2hr milling by basket mill, and 20 min homogenizing by a homogenizer. After flow curves and SEM images investigation in order to evaluation better dispersion of titanium dioxide particles, 6 paints were synthesized based on the specified formulations with 28.29% titan particles (micro plus nano titan). In order to evaluate solid material sameness in the coatings, the volatile content matter and volume non-volatile matter test were used. The solid content in the volatile content and volume non-volatile content test were 66.67% and 48 to 50%, respectively. For applying nano particles in the coating samples, 2 methods were used: 1- nano particles and micro particles exist simultaneously in one paste (combined paste), 2- and another nano particles and micro particles exist separately in the two pastes, and then added to vehicle paint. After that, primer and paint applied to the galvanized steel with 21 to 23 micrometer thickness and were cured at 300 °C for 30 seconds, and then quenching in water. The coat sample with 60% homogenized combined (C4) showed the best properties [rheological, mechanical (solvent resistance, T-bend, impact, cupping, scratch, pencil hardness, adhesion by tape method, abrasion), and corrosion (salt spray and humidity)]. In addition, mentioned properties for coat sample with 60% non-homogenized combined (C3) was better than coat sample with 60% micro paste (C1) which is using industry. C3 and C1 have approximately same manufacturing process. The C3, C4 coating due to high viscosity, have difficult process-ability for applying in coil coating process, but coat sample with 20% homogenized nano and 60% micro (C6) have acceptable viscosity for coil coating process and better properties rather than C1 coatings, C6 is appropriate choice for replacement of C1 coating. The C3, C4 and C6 coating rather than other coatings in comparison with C1 coating have best properties. C3, C4 and C6 coatings increased 11.5%, 15% and 13.2% abrasion resistance, 2, 3, 3 units pencil hardness and 1, 2, 2 degrees adhesion strength in comparison with C1 coating. In impact test, crack size for C1 coating was about 0.2 to 0.5 millimeter that in C3, C4, C6 reduced to 0.2 millimeter. In salt spray test delaminating around scribe from 49 millimeter for C1 coating, reduced to 40, 41, 41 millimeter for C3, C4 and C6, respectively. Key words: polyester resin, nano titanium dioxide, melamine resin, basket mill, homogenizer