Phage-displayed peptide library has fueled the discovery of novel ligands for diverse targets. A new type of phage libraries that displays not only linear and disulfide-constrained cyclic peptides but moieties that cannot be encoded genetically or incorporated easily by bacterial genetic machinery has emerged recently. Chemical posttranslational modification of phage library is one of the simplest approaches to encode nonnatural moieties. It confers the library with new functionality and makes it possible to select and evolve molecules with properties not found in the peptides, for instance, glycopeptides recognized by carbohydrate-binding protein and peptides with photoswitching capability. To this end, we describe the newly emerging techniques to chemically modify the phage library and quantify the efficiency of the reaction with a biotin-capture assay. Finally, we provide the methods to construct N-terminal Ser peptide library that allows site-selective modification of phage.