Graphite nanoplatelets (GNPs) with different sizes were compounded with polyetherimide (PEId) by melt‐blending and precoating in order to investigate their effect on the electrical and thermal conductivity of the composites. Precoating compounding successfully created a percolated GNP network with an electrical percolation threshold at 1 wt%, while melt‐blended samples required concentrations over 5 wt% to achieve percolation. The precoated samples also show higher thermal conductivity than the melt‐blended ones. Smaller GNP particle size results in lower thermal conductivity. An improvement of 1,600% in thermal conductivity was recorded for precoated 10 wt% GNP‐15/PEId composite as compared to the neat polymer and a polynomial dependence on GNP concentration was found to predict thermal conductivity. POLYM. COMPOS., 34:2148–2153, 2013. © 2013 Society of Plastics Engineers