The present invention discloses a method and an apparatus for measuring blood pressures by using blood oxygen concentration and electrocardiography, and the method uses a blood oxygen concentration analyzer to measure a continuous change of waveforms of a blood oxygen cycle during a cardio arterial pulse and an electrocardiography to confirm the pulse period of the systolic pressure and diastolic pressure of the blood pressure and correspond the pulse period with the continuous change of waveforms measured by the blood oxygen concentration analyzer. The microprocessor computes the corresponding cross-sectional areas of the continuous change of waveforms and the pulse period measured by the electrocardiogram, and calculates the values of the systolic pressure and diastolic pressure by a preinstalled formula, so as to accurately measure the values of blood pressures (systolic pressure and diastolic pressure).