This book is based on my M. Tech work on prediction of flow patterns of gas-liquid flow in microchannel using probabilistic neural network (PNN). It also contains a brief description on PNN and steps to develop a PNN in MATLAB R2008a. The analytical models present in literature, to predict the gas-liquid flow patterns employs different physics involved. So, for each flow pattern there are separate models being developed. This present work is aim to develop a single PNN model for predicting the flow patterns. The advantage of using a PNN is the ability to predict without any detail knowledge and understanding of the physics involved.