||Microelectronic packages and high brightness light emitting diode (HB-LED) packages face a common thermal challenge in high power density applications. The high thermal resistance of conventional thermal interface materials (TIM) continues to be a key issue of the heat dissipation in high power packages. Carbon nanotube (CNT) is a promising substitute for the polymer based TIM. In this thesis, several kinds of CNT-TIMs are developed and optimized. Their thermal performances are characterized according to the American Society for Testing and Materials (ASTM) D5470 standard and are evaluated in HB-LED packages. A theoretical understanding of the phonon heat transfer mechanism in CNTs is essential for CNT-TIM design and optimization. The factors influencing the thermal conductivity of CNTs are studied to find out the directions of improving the thermal performance of CNT-TIMs. Two possible kinds of CNT based TIMs, CNT-composite-TIM and vertically aligned CNT-TIM (VACNT-TIM), are prepared and their thermal performances are compared. The subsequent research work focuses on developing desired VACNT-TIMs because the thermal performance of CNT-composite-TIM fails to meet the thermal management requirement in high power packages. A CNT synthesis method is presented to fabricate VACNT-TIM with desired diameters. The effect of the CNT diameter on the thermal performance of TIM is studied. Plasma enhanced chemical vapor deposition (PECVD) is adopted to fabricate VACNT-TIM with high quality. The thermal performance of VACNT-TIM grown with PECVD is proved to be higher than that of VACNT-TIM fabricated with thermal CVD. A methodology to synthesize well aligned VACNT-TIM with high density on aluminum alloy heatsinks is developed for practical applications at low cost. In order to evaluate the effect of VACNT-TIM on the thermal and brightness performance of HB-LED devices, a novel packaging process is proposed. Some of the developed CNT-TIMs are applied in HB-LED packages and their output light power is measured. It is demonstrated that the VANCT-TIM helps to improve the brightness performance of the HB-LED packages.