Laser beam deflection has been used to observe thermal and sound wave or shock wave generated near the ablation threshold.The laser ablation thresholds of different metal samples in dry and moist atmospheric conditions have been detected by using a continuous porbe laser beam parallel to the surface of samples.Ablation threshold is 1.0 J/cm~2 for pure Cu and 0.9 J/cm~2 for brass and 0.68 J/cm~2 for Al in dry environment,1.0 J/cm~2 for pure Cu and 0.7 J/cm~2 for brass and 0.38 J/cm~2 for Al in moist environment.A comparison of the thresholds of different samples in dry and moist atmospheric conditions is made.The experimental results indicated that moisture indeed causes the reduction of ablation threshold to different extent for the metal samples we investigated.Results show that,ablation threshold reduces by 22 % for brass and 44 % for Al and does not reduce for pure Cu.