In NaZn13 type La(Fe,Si)13 compound, the phase transition nature varies from the first order to the second order, the cell volume contracts, the saturated magnetization decreases and the Curie temperature increases with increasing Si content. In this paper, the relation between the Curie temperature and the cell volume is investigated systematically by introducing the interstitial carbon atoms, which is an efficient method to control the cell volume and the Curie temperature. It is found that the relation between the Curie temperature and the cell volume is consistent with the Jaccarino-Walker model, in which only 5% or less 3d electrons are considered as the itinerant electrons and the others are regarded as the localized ones. With the polarized itinerant electrons used as a medium, the interaction between the 3d localized electrons is similar to Ruderman-Kittel-Kasuya-Yosida interaction, whose sign and magnitude oscillate periodically with distance. The number of the itinerant electrons of the La (Fe,Si)13 increases with the increase of Si content. The Curie temperature is dependent on both the cell volume and the number of itinerant electrons.