Thermal management of Li-ion batteries utilizing internal cooling method is the promising way to keep these batteries in an appropriate temperature range and to improve the temperature uniformity. In this study, three-dimensional transient thermal analysis was carried out to investigate the effects of size of embedded microchannels inside the electrodes on the thermal and electrical performances of a Li-ion battery cell. Based on the ratio of the width of microchannels to the width of the cell, different cases were designed; from the ratio of 0 (without any microchannels) to the ratio of 0.5. The results showed that increasing the size of the microchannels from the width ratio of 0 to the width ratio of 0.5 can reduce the maximum temperature inside the battery cell up to 11.22 K; it can also improve the temperature uniformity inside the battery cell. Increasing the electrolyte flow inlet temperature from 288.15 K to 308.15 K can enhance the temperature uniformity inside the battery and the cell voltage up to 33.20% and 2.79%, respectively. Increasing the electrolyte flow inlet velocity from 1 cm/s to 10 cm/s can reduce the maximum temperature inside the battery cell up to 8.09 K.