This paper presents a neutronics optimization study of a supercritical CO2-cooled micro modular reactor (MMR). The MMR is a fast-spectrum reactor designed to be an extremely compact, integrated, and truck-transportable reactor with 36.2-MWth power and a 20-year lifetime without refueling. The reactor uses a drum-type primary control system and a single absorber rod located at the core center as the secondary ultimate shutdown system. In order to maximize the fuel inventory in a compact fast reactor, hexagonal fuel assemblies are adopted in this work. We compare two types of MMR: One is using U15N fuel, and the other one is based on UC fuel. In addition, the minimization of the core excess reactivity to less than 1 dollar is also achieved in this study by a unique application of a replaceable fixed absorber in order to enhance safety of the MMR core by preventing the possibility of a prompt criticality accident. Moreover, the required number of primary control drums is also reduced through minimization of the excess reactivity. Several important safety parameters such as control rod/drum worth, reactivity coefficients, and power peaking factors are also characterized as a function of core burnup. The neutronics analyses and depletion calculations are all performed using the continuousenergy Monte Carlo Serpent code with the latest evaluated nuclear data file (ENDF/B-VII. 1) library. Copyright (C) 2016 John Wiley Sons, Ltd.