Abstract: The present invention relates to a method for preparing a bifunctional chelator for lanthanide. The method comprises the steps of providing a starting material which has an amino and carboxyl group; protecting the amino with an amino protecting group and the carboxyl with a carboxyl protecting group to produce a protected compound; reacting the protected compound with cyclen to generate a monoalkylated cyclen; reacting the monoalkylated cyclone with an activated compound to generated tetra-alkylated cyclone; removing the amino protecting group with a first protecting group removal reagent; and removing the carboxyl protecting groups with a second protecting group removal reagent to yield a bifunctional chelator having three more carboxyl groups and one or more amino groups.