Abstract: As the cornerstone of the development of the computational tool for the coupled neutronics and thermal-hydraulics analysis dedicated to the micro gas-cooled reactor, neutronic diffusion solver was developed in this work based on the generic 3D finite-element CFD code COMSOL Multiphysics by taking advantage of its equation-based modeling functionality “coefficient form PDE interface”. The steady-state and transient neutronic diffusion equations are solved using the eigenvalue solver and transient solver of this code, respectively. The neutronic diffusion solver developed in this work was firstly validated against two benchmark problems, i.e. 2D-TWIGL and 3D IAEA PWR problems respectively. The comparison of the calculation results of this solve with the reference values offered by the corresponding benchmark problems is satisfactory. Thereafter, this neutronic diffusion solve was applied to the micro gas-cooled reactor criticality analysis, with the group constants (including two-group and 25-group) generated by the Monte-Carlo code RMC. The calculation results of neutronic diffusion were also compared with the reference values derived by the Monte-Carlo transport calculations using the RMC code based on both continuous-energy-spectrum and multi-group methods. In general, the results obtained by solving both the two-group or 25-group neutronic diffusion equations, such as effective multiplication factor and three-dimension power distribution, can match well with the corresponding multi-group Monte-Carlo simulation. Whereas when comparing with the continuous-energy-spectrum MonteCarlo results, adopting the 25 energy groups is able to gain better accuracy in contrast to the two energy groups, indicating that the energy-group division has a significant impact on the neutronics for the micro gas-cooled reactor. The accuracy of prediction can be improved by adopting a more sophisticated energy group structure. However, the required computational source and time will dramatically increase with the number of energy groups.