In this work, a nanocomposite of Zr-trimesic acid MOF (MOF-808) with carbon nanotube (CNT) was synthesized through an in situ formation of MOF-808 on the activated CNT. The synthesized materials were characterized by powder X-ray diffraction, transmission electron microscopy, X-ray photoluminescence spectroscopy, Brunauer-Emmett-Teller, Fourier transform infrared spectroscopy, and Raman spectroscopy. The protein compatible nature with high surface area and electrocatalytic ability of MOF-808 was utilized to construct an immunosensor for ultra low-level detection of the ovarian cancer biomarker, carbohydrate antigen 125 (CA 125). The mutual benefit of each constituent of the MOF-808/CNT composite was capable of producing highly enhanced electrochemical properties. A glassy carbon electrode modified with MOF-808/CNT was used as a platform to fabricate a label-free electrochemical immunosensor. The antibody binding sites of MOF-808/CNT were enriched by functionalization with streptavidin. The immunosensor exhibited two linear determination ranges of 0.001-0.1 and 0.1-30 ng·mL, and the calculated limit of detection was 0.5 pg·mL (S/N 3). The immunosensor showed excellent reproducibility and selectivity. The patient serum sample analysis was cross-verified with the electrochemiluminescence method with a relative error of 105-110%.