Microbubble formation from a capillary tube surrounded by a co-flowing liquid (CFL) in a mechanically assembled microfluidic chip is investigated. The influences of gas pressure and liquid flow rate on the growth and detachment of the target microbubble are studied both experimentally and theoretically. A phenomenological theoretical model which consists of an expansion stage and a detachment stage is proposed based on a previous study to predict the detachment volume and formation time of microbubbles. The detachment distance is investigated experimentally in detail and later applied to the theoretical model. A MATLAB/Simulink simulation model is built for the solution of the proposed theoretical model. Model predictions correlate well with the experimental data in the present study.