In order to measure the relative change of muscle oxygenation non-invasively, dynamically and directly, a portable monitor based on near infrared spectroscopy (NIRS) was developed. The monitor consists of several function modules, including 735 nm, 805 nm and 850 nm integrated three-wavelength light emitting diode (LED) light source, LED driver, integrated detector, amplifier and filter, A/D sampling circuit, single-chip microcomputer and laptop. The distance between light source and detector is 3 cm and the photon migration depth in tissue is approximately 1.5 cm. The monitor is portable with low dark noise and good long-term stability. The relative change of muscle oxygenation measured by the monitor was in accordance with the real physiology status in the cuff ischemia experiment, verifying the performance of the monitor for living muscle. Two inflexions referring to an accelerated fall and a leveling-off phase in the muscle oxygenation index, respectively, were observed in in vivo incremental intensity exercises. Significant correlation was found between the first inflexion and the ventilatory threshold which was identified by the gas exchange measurement. These results demonstrated that the monitor can be used to detect the local lactate threshold of the measured muscle and reflect the changes of oxygen index in local muscle for in vivo exercises. The monitor may provide a meaningful approach to evaluate the subject’s oxidative capacity effectively.