Nanobodies have been extensively demonstrated in biomedical imaging and therapy. However, nanobody probes often suffer from rapid renal clearance due to its small size. Herein, we reported a recombinant nanobody with a 200 amino-acid polypeptide chain consisting of Pro, Ala, and Ser (PAS) at the C-terminal, which can be easily expressed in Escherichia coli with a high yield. The PASylated nanobody was functionalized with a fluorescent dye and the cell labeling properties were characterized by flow cytometry and confocal microscopy. In vivo fluorescence imaging indicated that the PASylated nanobody showed comparable blood circulation time, but ∼1.5 times higher tumor targeting ability as compared to the PEGylated nanobody of comparable molecular weight. Our findings demonstrate that nanobody PASylation is a promising approach to produce long-circulating nanobody probes for imaging and therapeutic applications.