Thermoplastic polymer such as high density polyethylene (HDPE) can be troublesome to manufacture due to its high viscosity of molten resin. To improve the manufacturability, polymer additive is a preferred method due to its low cost compared to changing the die geometry, process control setting and molecular weight of the resin. However, the influence of the additive to the mechanical and physical properties of HDPE was not fully understood. Therefore, the aim of this paper was to evaluate the effects of fluoropolymer and calcium stearate on melt flow index (MFI) and mechanical properties of HDPE. Twin screw extruder was used for compounding and injection molding was employed to produce the specimens. MFI, tensile and flexural tests were carried out according to their respective ASTM standards. An improvement of 20 in MFI was observed when 0.1 of fluoropolymer was added to HDPE. Adding calcium stearate to the mixture did not make any significant impact to MFI and mechanical properties. Tensile properties of HDPE remained almost the same with and without the additives. However, flexural property of HDPE reduced quite significantly with the additives. © 2018 Author(s).