The effect of magnetism on the thermoelectric (TE) transformation efficiency has recently attracted a lot of attention. A CrI3 monolayer is a two-dimensional (2D) ferromagnetic (FM) semiconductor with a Curie temperature of 45 K. In this work, we employed first-principles calculations within the framework of density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method and Landauer-Buttiker theory to study the effect of magnetism on the TE performance of a CrI3 monolayer. The stability, electronic structures, density of states (DOS) and TE parameters of a CrI3 monolayer are calculated. Our calculation results indicate that the TE performance of a CrI3 monolayer in a FM state is superior to that in a non-magnetic (NM) state. Namely, magnetism is beneficial to improving the TE performance. To further investigate the physical mechanism, the phonon group velocity, the electronic and phonon transmission spectra and the effective mass of a CrI3 monolayer in FM and NM states are analyzed in detail. For a CrI3 monolayer in a NM state, the maximum ZT value at 40 K is 0.09 and 0.16 for p-type and n-type doping, respectively. Relative to that in a NM state, the maximum ZT of a CrI3 monolayer in a FM state is largely improved, and can reach 0.23 and 1.58 for p-type and n-type doping. Our research provides a valuable reference by showing that magnetism is a possible factor for improving the TE efficiency.