Methyltetrazine-PEG8-NHS Ester is a PEG derivative containing a tetrazine group and an NHS ester. The hydrophilic PEG spacer arm improves water solubility and is transferrable to the labeled molecule, reducing aggregation of labeled proteins stored in solution. The NHS ester can react specifically and efficiently with primary amines such as the side chain of lysine residues or aminosilane-coated surfaces at neutral or slight basic conditions to form a covalent bond.