An investigation was carried out to study the effects of hydrothermal a carbonization on various agricultural wastes. A dual chamber reactor was designed to carry out the hydrothermal and vapothermal carbonization processes.From the experiments, the highest energy densification achieved was 1.280 invapothermal carbonization and 1.145 in hydrothermal carbonization for oil palm empty fruit bunches. Comparison of the hydrochar produced from both processes shows that vapothermal carbonized materials have higher heating values and higher fixed carbon contents as compared to those from hydrothermal carbonization process. Volatile matter of vapothermal carbonized materials have drops about 20% while their fixed carbon content are increased by 70-95% compare to the raw feedstock. With the supports of the SEM pictures and DTG curves of the products, it was concluded that the reaction mechanism during the hydrothermal and vapothermal carbonization are different. Devolatization was found to be the main reason that increases the overall energy content in vapothermal carbonization, while formation of 2,5-HMF which gives a sharp peak at around 340°C in the DTG curves was suggested as the reason that led to the increase in energy content in hydrothermal carbonized materials. Other than that, this project also showed that lower water to biomass ratio led to higher energy densification. A comparison of torrefaction, hydrothermal and vapothermal carbonization processes also showed that hydrothermal and vapothermal carbonization are capable to suppress the deposition of tar on the reactor wall during the carbonization process