Myoglobin was hydrolyzed by microwave-assisted weak acid hydrolysis with 2% formic acid at $37^{circ}C$, $50^{circ}C$, and $100^{circ}C$ for 1 h. The most effective hydrolysis was observed at $100^{circ}C$. Hydrolysis products were investigated using matrixassisted laser desorption/ionization time-of-flight mass spectrometry. Most cleavages predominantly occurred at the C-termini of aspartyl residues. For comparison, weak acid hydrolysis was also performed in boiling water for 20, 40, 60, and 120 min. A 60- min weak acid hydrolysis in boiling water yielded similar results as a 60-min microwave-assisted weak acid hydrolysis at $100^{circ}C$. These results strongly suggest that microwave irradiation has no notable enhancement effect on acid hydrolysis of proteins and that temperature is the major factor that determines the effectiveness of weak acid hydrolysis.