High density polyethylene (HDPE)/Organoclay nanocomposites with different compositions (1, 3 and and 5 wt. %. Closite 15A) were prepared using a co-current twin screw extruder in 190¢ªC and 200 rpm. Polyethylene-grafted-maleic anhydride (MPE) was used as a compatibiliser for this system. The blending was done via two different processing conditions of direct blending and also preparing a master batch of organoclay and MAP first and then introducing it to HDPE. The rheological properties of samples was studied by a Rheometric Mechanical Spectrometer (RMS) and mechanical properties of nanocomposites were investigated via tensile and impact tests. Rheological results showed that the storage and loss modulus of all samples are increased by introducing and increasing the amount of organoclay. Also the mechanical tests revealed that the Young¡¯s modulus of all nanocomposites are higher than neat HDPE, while the impact strength of all samples are lower than it. Also nanocomposites containing 3 Wt. % of organoclay and 9 Wt. % of MPE showed the highest tensile strength of 20 % higher than neat HDPE. Finally it was shown that the processing conditions do not affect on properties of samples significantly. Finally the XRD results showed the intercalation of organoclay platelets in the nanocomposite samples too. Keywords: Polyethylene, Organoclay, Nanocomposites, Rheological, Mechanical properties