The characteristics of microbubble generation were investigated experimentally at pressures up to 6 MPa under methane–water bubbly flows driven by a submersed centrifugal pump in a closed flow loop. The focused beam reflectance measurement was utilized for in-situ microbubble sizing at the pump downstream. The influence of system pressure was evaluated for turbulent shear rates between 4.5×10³ and 23×10³ s⁻¹ at 298 K and a void fraction of 0.016. The microbubble number density was found to increase significantly with increasing pressure; whereas, the median bubble size fell within the magnitude of the Kolmogoroff microscale. The transient size distribution functions indicated that turbulent shear coalescence plays a significant role at high shear rate above 10⁴ s⁻¹. It was also suggested that the jetting pump flow at higher pressures could enhance microbubble stability.