Abstract: With regard to the force characteristics at the point of departure and lift-off, the momentum equations of bubble were simplified according to the result of force balance analysisThe criteria of bubble departure and lift-off point were proposed as wellThe predicted results were compared with the experimental results of Thorncroft, the error of bubble departure diameter and lift-off was within ±20%.In up-flow boiling, higher wall superheat or lower mean velocity led to increasing bubble departure diameterIt was generally the same in the case of down-flowBecause of opposite effect of drag force and buoyancy force on departing bubbles, there was no bubble departure of the flow at the velocity ranged from 02—03 m·s-1In up-flow boiling, the same direction of the bubble sliding on heated surface and the main flow led to reduced velocity difference between bubble and main liquid and greatly increased lift-off diameterIn down-flow boiling, the bubble lift-off diameter also increased with lower mean velocity and higher wall superheatHowever, as compared with departure diameter, the bubble lift-off diameter did not greatly increase with higher wall superheat.