Monitoring of cellular cultures oxygen partial pressure (pO2) in liquid phase with a fluorescence quenching based fibre optic oxygen sensor is reported. The sensitivity of the fibre optic oxygen sensor was evaluated in gas phase and calibrated in liquid phase before the monitoring of cell culture pO2 with a luminescence phase detection system. In the experiment of cell culture pO2 monitoring the porcine kidney (PK-15) cells were used as the cell samples and the experimental results showed that the sensor is suitable for monitoring of pO2 in gas and liquid phase. And we found there are different measured pO2 level in liquid phase for different cell densities in cell culture at ambient air environment, which has a PO2 of 21kPa corresponding to an oxygen concentration of 21%. Under the condition of low pO2 in gas phase and high cell density, the cell metabolism could shift pO2 in the liquid phase much lower than that would be from the O2 gas/air equilibrium coefficient, which indicates that the cellular oxygen uptake rate exceeds the oxygen diffusion rate. Therefore there is a need for direct pO2 monitoring in cell culture, particularly in static high density cell culture systems, in which oxygen mass transfer is limited.