The critical heat flux (CHF) phenomenon at low pressure and low mass velocity (LPLV) conditions is important in relation to reactor accident conditions, inherent safety reactors and various research reactors, whereas available experimental data are very limited and the mechaisms are not well understood. In this work, we have collected the reported experimental data and analyzed them to identify the CHF mechanisms and to develop a reliable set of CHF correlations. Furthermore we have performed the experiments to extend the available data. The examination of the CHF data indicates that the CHF condition occurs by the flooding mechanism at extremely low mass velocities and by the transition from slug(or churn) to annular flow at low mass velocities. The flooding mechanism gives considerably lower CHF values than conventional pool-boiling CHF correlatons. The slug-to-annular flow transition causes flow stagnation and/or flow reversal which seem to induce the CHF condition easily for low flow. Additionally, we derived the criteria for the region of the flooding-limited CHF in terms of channel diameter and mass velocity. We made the CHF correlations for circular tubes and annulus channels based on this flow regime transition criterion, and compared them with experimental data. The correlations show good performance in prediction of CHF at LPLV conditions. Experiments have been performed for a 6 mm-I.D. circular tube at atmospheric pressure as the first phase. The experiments give slightly higher CHF values than previous works. The experimental results are discussed briefly, because further experiments are being performed.