Protein tyrosine phosphatase 1B (PTPIB) plays a major role in regulation of dephosphorylation of insulin receptor. The present study was designed to elucidate the effect of Arg 221 in catalytic site. Mutants of Arg 221 were carried out using the computer program HyperChem Release 7.0. The variation of Arg 221 had significant effects on the interaction of substrate with protein. When Arg 221 mutated to basic residues, tlte electrostatic interaction energies betwoon substrate and protein increased but the contribution of catalytic residues decreased. Other mutations caused decrease in total electrostatic interaction energies. Our data suggested that mutations of Arg 221 to basic residues decreased the catalytic activity but increased the binding affinity for substrate, and mutations to others would decrease both catalytic and binding abilities.