A simple, selective and sensitive spectrofluorimetric method has been reported in this paper for the assessment of procalcitonin (PCT) as a biomarker in serum samples of cancer patients by using optical sensor Tb3+-hydrochlorothiazide (Tb3+-HCTZ) complex. The method based on the quenching of the luminescence intensity of Tb3+-HCTZ complex by addition of different concentrations of PCT. The optical sensor was synthesized and characterized by absorption and fluorescence measurements. The performance of the designed sensor is determined through monitoring the quenching of the fluorescence intensity at 545 nm by (PCT) after excitation at 320 nm and pH = 7 in DMSO. The calibration of this method was performed in the concentration range of (3.37–100.62 ng/mL) with a limit of detection (LOD) and quantitation limit of detection (LOQ) are (0.038 and 0.114) ng/mL, respectively and a correlation coefficient of (0.98). This method was practical, simple and relatively free from interference effect. It was successfully applied to measure PCT in samples of human serum and from this method we can assess some biomarkers of cancer related diseases in human body.