The growing process of the individual microbubble in an open rectangular capillary microgroove was theoretically analyzed in this study. Several correlations of bubble growth rate for pool boiling were proved not available for microgroove boiling. A theoretical model based on thermal equilibrium and force balance was developed in this article. The growing process of the individual microbubble was divided into three stages: initial growing stage, normal and axial confined ellipsoidal growing stage, and axial subcylindrical growing stage. Growth period and volume increment of the micro vapor bubble were analyzed. The calculation results indicate that the growth of the micro vapor bubble is confined by the geometric structure of the microgroove. Comparison of the results between calculation and experiment shows that the correlation is available to predict the bubble growth rate for boiling in microgrooves.