In this Letter, the proton transfer in the chromophore site of green fluorescent protein is studied by the B3LYP method with 6-31G(d,p) and 6-31+G(d,p) basis sets. A concerted proton transfer is observed between the chromophore and its neighboring residues (water, His148, Ser205, Glu222). It involves simultaneous motion of three protons. The transition state for hydrogen transfer is located with the transition barrier estimated to be slightly below zero with Zero-Point correction. This is a characteristic feature of low-barrier hydrogen bonds. His148 plays an important stabilizing role on the transition state and thus controls the proton shuttle. Solvation effects based on self-consistent reaction field are found to stabilize the deprotonated form more than its corresponding neutral form.