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In this paper we discuss the linear relations between the F layer critical frequency (foF2) over Indonesian region and T-index based on the semi-empirical model contained in the ASAPS (Advanced Stand-Alone Prediction System) prediction software. The foF2 values for Indonesia region is represented by its value for Biak, Kototabang, Pontianak, and Sumedang stations. By simulations using ASAPS with T-index = 0 and T-index = 100, we were obtained the median foF2 and the coefficients of linear equations (a and b) which relates the average of the median of foF2 during 24 hours () and T-index. As a comparison, we have determined the coefficients a and b which relates and the sunspot number, 12 R . The results show that coefficients of the linear equation between and T-index, and for linear equation between and 12 R are very closed, such that the T-index equation is consistent. The effect of the solar position to the foF2 change has been considered in the equations. The T-index equation is characterized by the coefficients a' and b' which are vary for each month and station.