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Global solar radiation, which passes through the atmosphere and arrives on earth's ground, is composed of two constituents, that is, beam and diffuse radiation. It is agreed that the beam radiation is the only effective part for use of solar energy in concentrating either photovoltaic or solar thermal energy application, which concentrates the incident sunlight to attain high performance of either electrical power or high-temperature thermal energy generation, respectively. Information of beam radiation quantity incident upon an absorbing surface is, thus, a prerequisite for the performance evaluation of concentrating optic device. However, very few data of diffuse fraction, defined as the ratio of diffuse radiation to global radiation, is available in Taiwan. A long-time monitoring investigation in the main island (Taiwan proper) and one offshore islet (Penghu) were conducted over recent years. It was found that the annually averaged diffuse fractions, calculated with the time duration from sunrise to sunset, in the east (Taitung) and west (Tainan) sides of the main island were about 0.49 and 0.53, respectively, while about 0.50 in the offshore islet (Penghu). Seasonal variations due to geographical/topographical effects on diffuse fraction are also reported and discussed. It is found that Northeast Monsoon has remarkable effects on increment of diffuse fraction in Taitung and Penghu, while insignificant effect in Tainan. [ABSTRACT FROM AUTHOR]