Diesel exhaust fluid, DEF, (32.5 wt.% urea aqueous solution) is widely used as the NH3 source for selective catalytic reduction (SCR) of NOx in diesel aftertreatment systems. The transformation of sprayed liquid phase DEF droplets to gas phase NH3 is a complex physical and chemical process. Briefly, it experiences water vaporization, urea thermolysis/decomposition and hydrolysis. Depending on the DEF doser, decomposition reaction tube (DRT) design and operating conditions, incomplete decomposition of injected urea could lead to solid urea deposit formation in the diesel aftertreatment system. The formed deposits could lead to engine back pressure increase and DeNOx performance deterioration etc. The formed urea deposits could be further transformed to chemically more stable substances upon exposure to hot exhaust gas, therefore it is critical to understand this transformation process. In this work, lab experiments were designed to simulate urea deposit formation process by treatment of pure urea at 100-250 °C for various durations up to 100 h. The effect of water vapor was also investigated. The lab formed urea deposits were subsequently characterized by thermogravimetric analysis (TGA) and then compared to the TGA curves of pure reference chemicals of urea, biuret, cyanuric acid (CYA) and ammelide. The TGA results indicate that 1) below urea melting temperature (130 °C), the formed urea deposits are still primarily urea; 2) in the temperature range 130-190 °C, urea transforms to a combination of urea, biuret and CYA; 3) above biuret melting temperature (190 °C), CYA is primarily formed with some features of ammelide. Above 200 °C exposure, urea experiences fast chemical transformation via urea decomposition, biuret formation followed by subsequent biuret decomposition, and turns into CYA in a short time. A model was also built and validated to estimate the main chemical composition of urea deposit.