The vane compressor used for automotive air conditioners maintains contact between a vane tip and a cylinder wall by centrifugal force and backpressure acting on the vane under steady-state operation. However, under start-up operation, the backpressure is not sufficient to maintain the vane tip contact, and a chattering phenomenon which causes noise and partial wear occurs. We analyzed vane behavior under start-up operation by applying an equation of motion to the vane. We also observed the vane behavior after start-up by visualization in the compressor cylinder. The influences of oil shearing force, inertial force due to rotor acceleration and the backpressure on the vane behavior during start-up operation were clarified. The validity of the analysis was confirmed by comparison of calculated results with experimental ones.