Photosynthetically active solar irradiance (PAR, 400--700 nm) data measured with a rotating shadowband radiometer at Yonsei University, Seoul (37.57¿N, 126.97¿E) from 1996 through 1997 have been analyzed to investigate statistical features in components of the horizontal PAR and the dependence of the diffuse PAR on total optical depth (TOD) for visible range. Based on our measurements, it is found that the seasonal and sky conditional percentages of the global PAR for the direct and diffuse components depend considerably on the magnitude of the TOD. Annual mean diffuse PAR at Seoul appears to be 51% of the global-horizontal PAR (5.12 ¿ 1.27 M J m-2 d-1) for all-sky conditions with a TOD of 1.02 ¿ 0.32. However, the diffuse PAR are 39% of the global PAR (6.10 ¿ 2.27 M J m-2 d-1) under clear-sky conditions (cloud amount ≤ 2/10) with a TOD of 0.47 ¿ 0.23 and 73% of the global PAR (2.99 ¿ 2.07 M J m-2 d-1) under cloudy sky conditions (cloud amount ≥ 8/10) with a TOD of 1.80 ¿ 0.45, respectively. Simple statistical models are presented for the determination of direct and diffuse-horizontal PAR as a function of TOD, which is fitted by a curvilinear regression at a given solar zenith angle (SZA). Overall, there was reasonable agreement between predicted and measured values of two PAR components. On the basis of the statistical models for direct and diffuse PAR, we found that the four parameters of the maximum diffuse PAR, the TOD corresponding to the maximum diffuse PAR, the PAR and TOD when direct and diffuse PAR have the same value, decrease all with increasing SZA. It is of interest to note that these all four parameters can be expressed as simple linear functions of SZA.