For achieving cost effectiveness in solar power generation, ultra-high concentrator photovoltaic (UHCPV) system operating at 1800 suns is highly recommended in order to minimize the usage of semiconductor material. Although sunlight focusing can be accomplished via two-stage concentrator consisted of non imaging dish concentrator and an array of crossed compound parabolic concentrator lenses, the thermal management of concentrator photovoltaic (CPV) cells remains as a crucial problem. The objective of this study is to optimize the configuration of multiple-channel heat sink with the best design in thermal performance so that the temperatures of CPV cells are below 100 degrees C even operating under ultra-high concentrated sunlight. Comprehensive analysis has been carried out via computational fluid dynamics (CFD) simulation to study thermal performance of heat sinks for different configurations with various fin thicknesses and fin heights. To emulate the real case, optical analysis has been carried out via ray-tracing method to simulate the solar flux distribution and input solar power illuminated on receiver so that the results can be fed into the CFD modeling. The heat sink with configuration of 1 mm fin thickness x 20 mm fin height (1 x 20) was found to be the most optimized design in which the CFD simulation has shown the lowest values for both average temperature of CPV cells and maximum temperature difference between CPV cells. By optimizing the average water velocity at 0.6 m/s, the heat sink with the configuration of 1 x 20 can maintain the CPV cells operating at 91.4 degrees C under solar concentrator ratio of 1800 suns and direct normal irradiance of 1000 W/m(2). Through the optimization of the thermal performance, the UHCPV system can produce the net electrical output power of 4064 W at power conversion efficiency of 31.8%. (C) 2017 Elsevier Ltd. All rights reserved.