We use two available methods, the Duhem-Gibbs relation and the entropy formula in terms of particle phase space distributions, to calculate the entropy in a quark combination model. The entropy of the system extracted from the Duhem-Gibbs relation is found to increase in hadronization if the average temperature of the hadronic phase is lower than that of the quark phase. The increase of the entropy can also be confirmed from the entropy formula if the volume of the hadronic phase is larger than 2.5-3.0 times that of the quark phase. So whether the entropy increases or decreases during combination depends on the temperature before and after combination and on how much expansion the system undergoes during combination. The current study provides an example to shed light on the entropy issue in the quark combination model. The quark combination model [1, 2] (QCM) or its variants such as the quark recombination or coalescence model are used to describe multi-hadron processes in high energy electron-positron, hadron-hadron and nucleusnucleus collisions. Recently they are successful in explaining the data at Relativistic Heavy Ion Collider (RHIC), which showed that the combination is more preferable than the fragmentation for the hadronization mechanism in intermediate transverse momentum region in heavy ion collisions at high energies [3][4][5]. The basic idea of the QCM is that hadrons are combined from quarks and anti-quarks, i.e. two quarks merge into a meson (a 2-to-1 process) and three quarks into a baryon (a 3-to-1 process). One of the most frequently asked questions about the validity of the QCM is: does the entropy decrease in such 2-to-1 and 3-to-1 processes? The question arises naturally from the common sense that the entropy is normally reduced in a particle number decreasing process. The entropy problem is essential to nucleusnucleus collisions since the issue of the entropy production is closely related to thermalization and has been the concern of the researchers for many years. There have been previous attempts or arguments about the entropy in the QCM-like models in literature [6][7][8][9], but no systematic investigation has yet been made. In this paper we will address the entropy problem in the course of the quark combination in heavy ion collisions. As an example we will use a specific version of the QCM which is developed by the Shandong group [10][11][12][13]. The advantage of the Shandong QCM is its exclusive nature and simplicity, i.e. it can give all hadrons in an event. Considering that the entropy can be regarded as a measure of degrees of freedom, the exclusive nature of Shandong QCM is suitable and applicable to the entropy problem.The building blocks for the QCM are constituent quarks and anti-quarks of the light flavors u,d and s with the momentum distributions f i (p) ≡ f i (y, p T ) [14][15][16]. All hadrons come out with fixed momenta from combination of their constituent quarks. Note that our QCM (or any of QCM-like models) does not deal with how the constituent quarks acquire masses...