Penetration of photovoltaic (PV) generation system into the power system may have some negative effects on stable operations of the power system; for example, on frequency control. Therefore, investigation of the shortterm fluctuations of PV generation is important as a precaution for further PV penetration. Since PV generation is approximately proportional to the incident solar radiation, this paper develops static and dynamic characteristic models for short-term fluctuations of global solar radiation. The static characteristic model is a set of standard deviations which have been statistically estimated based on past observed data. The dynamic characteristic model consists of autoregressive models which are designed for actually observed time-sequential short-term fluctuation data. In both models, the clearness index is used to eliminate seasonal variation of solar radiation.