Biomass is the third largest primary energy resource in the world after coal and oil. Due to its huge potential, the renewable and the corresponding positive role for CO2 reduction, the use of carbonized biomass for energy purpose is expected to increase. The carbonized biomass comes from agricultural waste and forest by-product. Carbonized plant and carbonized coconut cell biomass were mixed with water to study the possibilities of slurry preparation as a carbonized biomass water mixture (CBWM). Beside that, the upgraded coal by an upgraded brown coal (UBC) process was also studied to produce a UBC water mixture (UBCWM) with high coal concentration. The rheological characteristics of CBWM and UBCWM have been conducted by using a stress controlled rheometer. The results indicate that the maximum concentrations of the carbonized plant, carbonized coconut cell biomass, and UBC were 35.9, 51.2, and 61.5 wt%, when respectively using 0.3 wt% of naphthalene sulfonic acid (NSF), polymethacrylate (PMA), and NSF as dispersing additives, and 0.1 wt% of carboxyl methyl cellulose (CMC) as a stabilizing additive.