Improving the thermal conduction ability of thermal interface materials (TIMs) can enhance heat transfer in electronics packages and reduce chip temperature. In the present work, multi-walled carbon nanotubes (MWCNTs) are used as additives for enhancing the thermal conductivity of the widely-used TIM silicone grease. The effects of the MWCNTs length and surface treatment on the silicone grease conductivity are studied. The results show that the MWCNT length has strong influences on the thermal performance of the grease with MWCNT additives. The equivalent thermal conductivity of the composite layer of the grease and the solid surfaces increases as the MWCNT length decreases. Only the length of MWCNTs is short enough, their addition can improve the thermal performance of the common silicone grease. Surface treatment on MWCNTs can improve the thermal performance of the grease. Through transmission electron microscope (TEM) observation it is found that after the modification of the short MWCNTs the MWCNT entanglement phenomenon in the mother silicone grease is weakened and the MWCNTs are dispersed more uniformly. The influence of the contact pressure on the thermal performance of the MWCNT TIMs is also investigated. The results show that the thermal resistance of the composite layer formed with different TIMs all decreases with the applied contact pressure, but the TIMs with the addition of the short and chemically treated MWCNTs are less sensitive to the contact pressure. (C) 2014 Elsevier Ltd. All rights reserved.