The monthly average values of the main solar activity indices such as Wolf numbers (W), 10,7 cm radio flux F 10,7 , 0,1-0,8 nm background, total solar irradiance and Mg II UV-index (280 nm core to wing ratio) were studied for 23 activity cycle. The correlation coefficients of linear regression for main solar activity indices versus F 10,7 were analyzed for every year of 23 activity cycle. All the correlation coefficients for 23 activity cycle have the minimum values in 2001. We also analyzed the connection between monthly average values of the most popular indices W and F 10,7 for 21, 22 and 23 solar activity cycles. The values of every year linear regression correlation coefficients for W versus F 10,7 show the cyclic variations with period closed to half path of so-called 11-year period of solar activity.KEYWORDS: Sun, solar activity indices, monthly average values variations, correlation coefficients of linear regression for main solar activity indices