Graphene nanoplatelets (GNPs) have exceptional electrical and mechanical properties that can be used as a filler for conductive polymer. However, the size of the GNP can affect the conductivity of the conductive polymer as well as its reliability, especially when it is subjected to a different type of loading during its applications. This study is conducted to showcase the effect of particle sizes of GNP as a filler on its conductivity and the reliability of the conductive polymer composites when subjected to mechanical fatigue stress through the bending test. In this work, two types of GNP filler sizes are considered, these being the 5µm (5M) and 15µm (15M) with an epoxy binder. The initial results show that 5M GNP-filled conductive polymer composites has 92.54% and 96.28% higher in bulk and sheet resistivity than 15M GNP conductive polymer. Following the cyclic bending test, the results show that the resistivity increases as the number of cycles increases due to cracks' formation. Other than that, it was found that the rate at which the resistivity increases within the 5000 cycles of bending for 5M conductive polymer is much lesser compared to that of 15M conductive polymer. The increment in bulk and sheet resistivity is 22.70% and 17.68%, respectively, for 5M, while 15M was found to be as much as 55.90% and 36.33%. The stability on the conductivity of the smaller size particle was discussed to be due to its area of surface contact after being bent through the cycles.