Hadron therapy, such as carbon and helium ions, is increasingly coming to the fore for thetreatment of cancers. Such hadron therapy has several advantages over conventional radiotherapyusing photons and electrons physically and clinically. These advantages are due to the differentphysical and biological characteristics of heavy ions including high linear energy transfer and Braggpeak, which lead to the reduced exit dose, lower normal tissue complication probability and theincreased relative biological effectiveness (RBE). Despite the promising prospects on the carbonion radiation therapy, it is in dispute with which bio-mathematical models to calculate the carbonion RBE. The two most widely used models are local effect model and microdosimetric kineticmodel, which are actively utilized in Europe and Japan respectively. Such selection on the RBEmodel is a crucial issue in that the dose prescription for planning differs according to the models. Inthis study, we aim to (i) introduce the concept of RBE, (ii) clarify the determinants of RBE, and (iii)compare the existing RBE models for carbon ion therapy.