Variability of the Photosynthetically Active Radiation (PAR) measured at the ground- based solar radiation monitoring station at the Chisinau site from 2004 to 2006 is analyzed. Series of days with observations for clear sky conditions was selected from the original PAR and solar radiation datasets to study relationship between PAR and global, diffuse and direct solar irradiance, and to evaluate conversion factor for converting PAR values measured with specific PAR sensor from photosynthetic photon flux to PAR in energy units. This selection is necessary to neglect the influence of loudiness upon the measured data. There was proposed a simple power type relationship approximating diurnal variability of PAR and depending on aerosol optical depth. There was demonstrated strong positive correlation between daily totals of PAR and global solar radiation both for clear sky and all-sky conditions, and respective linear equations between these parameters are proposed. Monthly mean values of daily ratios of PAR and global solar radiation for all-sky conditions show distinct seasonal variation with large values observed during cloudy months specific to fall and winter seasons. It is attributed to predominant absorption of solar radiation on aerosols, gases and in clouds occurring outside of PAR spectral domain. For very clear days of observations the growth of daily mean ratio of PAR and solar irradiance was due to increase of aerosol optical depth. There was supposed a simple linear relationship for evaluation of incoming PAR through direct and diffuse solar radiation by applying specific weighting coefficients. For all-sky and for clear sky conditions weighting coefficients amount to ~0.46 and ~0.59, respectively. Analysis of radiometric data acquired during one of the clearest day in time series of observation gives conversion factor which amounts to ~4.65. flux to PAR in energy units. This selection is necessary to neglect the influence of cloudiness upon the measured data. There was proposed a simple power type relationship approximating diurnal variability of PAR and depending on aerosol optical depth. There was demonstrated strong positive correlation between daily totals of PAR and global solar radiation both for clear sky and all-sky conditions, and respective linear equations between these parameters are proposed. Monthly mean values of daily ratios of PAR and global solar radiation for all-sky conditions show distinct seasonal variation with large value to predominant absorption of solar radiation on aerosols, gases and in clouds occurring outside of PAR spectral domain. For very clear days of observations the growth of daily mean ratio of PAR and solar irradiance was due to increase of aerosol optical depth. There was supposed a simple linear relationship for evaluation of incoming PAR through direct and diffuse solar radiation by applying specific weighting coefficients. For all-sky and for clear sky conditions weighting coefficients amount to ~0.46 and ~0.59, respectively. Analysis of radiometric data acquired during one of the clearest day in time series of observation gives conversion factor which amounts to ~4.65.