The damage and annealing behavior of Si implanted at room temperature with P2+ and P+ at different energies (5-600 keV) and intermediate dose (～1014/cm2) has been investigated. Experimental results show that the damage created by P2+ implantation is always greater than that of P+ implantation. The ratio of total displaced atoms of the target cuased by molecular and ND(mol)/ND(atom)reached a maximal value at 100 keV (P2+)and 50 keV (P+) after rapid thermal annealing, the carrier concentration profiles measured by spreading resistance measurement are also different for the P2+ and P+ implanted samples. We attribute essentially this phenomenon to the displacement spike, but the multiple collision effect and the interaction between two molecular fragments should be considered when the incident energy is high.