Ⅲ-Ⅴ multijunction solar cells are key components in a concentrator photovoltaic (CPV) system.The cell efficiency and characteristics have important impact on the system efficiency and energy output, and thus directly affect the levelized cost of energy (LCOE).With the emerging market of CPV technology, a number of Ⅲ-Ⅴ multijunction solar cell companies have emerged in the last few years.Because of complexity of multijunction solar ceils it is difficult to compare cell efficiency measured by different cell vendors under flash test.in order to achieve the lowest LCOE it is necessary to accurately evaluate cell performance in CPV systems.In this paper we characterized and compared multijunction solar cells from seven vendors via indoor flash test, outdoor on-sun test and spectral response measurement.Most vendors have achieved similar efficiency in both indoor flash test and on-sun test.The relative difference among different ceil vendors changes in two tests because of their different spectral responses and temperature coefficients.The spectral sensitivity of Ⅲ-Ⅴ multijunction cells leads to performance variation in the field that is not always well predicted by traditional indoor flash test or spotty on-sun tests.The results for different epitaxial designs and limitations of the test methodologies are presented.The comparative results lead to future design modifications for maximum energy output.