The heat generation characteristics of LixMn2O4 spinel as a positive electrode material were measured in situ during cycling of Li/LixMn2O4 cell, using an isothermal microcalorimeter in combination with a battery tester. The total heat generation rate is dependent on the concentration of lithium in LixMn2O4. By separating irreversible heat from the total heat rate, using,de interruption technique, the concentration dependence is shown to be mainly due to the reversible heat of the cell, which shows a cooling-heating-cooling-heating cycle, during charge/discharge. This thermal characteristic may be explained from the phase diagram of LixMn2O4, in agreement with recent reports on the phase-change behavior. The dependence of the thermal characteristics on the current level is weak at a low C rate, but increases steeply from C/3 to IC rate. Although voltage profiles of charge and discharge showed similar behavior, the range of discontinuity (between 4.1 and 4.0 V) is not identical for charge and discharge. While the voltage profile is affected by irreversibility (charge vs. discharge), the thermal profile is barely affected by it.