Coherent acoustic phonon (CAP) behaviors in GaN based materials were investigated using femtosecond transmission pump-probe measurements. By optical pumping, strong coherent acoustic phonon oscillation can be generated in InGaN/GaN multiple-quantum-wells with strong strain-induced piezoelectric fields. The induced optical transmission modulation was 10^3 times of the induced reflectivity modulation in pervious reports in other semiconductors. The magnitude and phase of the initialed CAP oscillations can be coherently controlled, demonstrated using another femtosecond pulse by controlling the pulse time de lay and intensity. Through the interferometry analysis based on a simple harmonic oscillator theory, the existence of phonon nonlinearity was revealed after calibration.