The aim of this study is to investigate the bubble formation in different liquids within three types of gas-liquid micro-mixer geometries, including a T-junction and two Y-junctions. The bubble shape, size and formation mechanism were studied under various experimental conditions such as flow rates of both phases, physical properties of liquid and mixer's geometries. A micro-Particle Image Velocimetry (mu-PIV) technique and a high speed camera were used to characterize and quantify gas-liquid flows. It was revealed that the bubble formation, in particular the bubble size, depends on the geometry of the mixing section. A correlation gathering numerous experimental data was elaborated for the estimation of bubble size. The influence of different parameters like flow rate ratio between two phases, mixer geometry and surface tension is well taken into consideration based on the understanding of bubble formation mechanism at microscale. This paper marks an improvement in the domain where no global correlations were available in T and Y-junction devices, and this correlation is then an useful tool for micro-mixer design.