We demonstrated coherent control of the 40-THz optical phonons in a single crystal diamond using a pair of 8-fs pulses and a pump and probe protocol. We detected the coherent phonon oscillation via change in transmitted light intensity of the probe pulse with heterodyne detection. The phonon amplitude was coherently controlled by changing the delay between two pump pulses from 230 fs to 270 fs with 0.5-fs precession. The control scheme was explained by interference between phonon states excited by each pump pulse. The results are well reproduced by the quantum mechanical model with two electronic states coupled with a harmonic oscillator.