This paper presents the effect of reservoir conditions, specifically temperature and pressure, on the rheological behavior and density of super lightweight completion fluid (SLWCF) for underbalanced perforation. In this study, fluid's density was measured at various temperatures and pressure ranging from 313.15 K to 393.15 K, and 0.1 MPa to 25 MPa, respectively. Meanwhile, fluid's viscosity was measured at temperature between 298.15 K to 373.15 K, and pressure range of 0.1 MPa and 4.48 MPa. In order to understand the effect of reservoir conditions to the density and viscosity of the fluid, experimental data were fitted to several density-/viscosity-temperature-pressure models and then the generated results were statistically evaluated. Based on the results, it is observed that the Tait-like equation was able to satisfactorily express the relationship between the density, pressure and temperature. The predicted density values based on the Tait-like equation are also in good agreement with the regressed model results. For the case of fluid's viscosity, it is found that both modified Mehrotra and Svrcek's and Ghaderi's equation were the best equation for viscosity prediction. Using these equations, it is statistically possible to predict the variation of fluid's density and viscosity over the wide range of pressure and temperature. Furthermore, it is also found that the predicted density and viscosity values are very close to the experimental data with very low deviation. This confirmed the reliability and accuracy of the prediction. This paper provides a novel data prediction of rheology and density of Saraline-based SLWCF at reservoir conditions for the purpose of underbalanced perforation. This result is essential as a tool for field engineers to roughly estimate the density and viscoplasticity of completion fluids as they subjected to reservoir conditions.