A grooved sidewall channel is employed in the evaporator section of a flat-plate micro pulsating heat pipe to enhance the thermal performance. The grooved sidewall channel is thermally optimized to maximize the evaporation heat transfer coefficient. For this, a mechanistical model for the liquid film evaporation in the two-phase flow boiling in a grooved channel is developed. According to the model, the groove depth has significant effect on the evaporation heat transfer coefficient of the liquid film evaporation, and the optimal groove depth is 50 µm. The optimization result is validated by experiments of flat-plate micro pulsating heat pipes with grooved sidewall channels which have different groove depths with each other. A flat-plate micro pulsating heat pipe is shown to have up to 64.1% higher thermal performance than the flat-plate MPHP with a bare channel.