The mechanical properties of 20CrNiMo steel and the effects of notch with different fillet radii on tensile properties and tensile fracture morphology were studied by tensile tests on smooth specimens and notch specimens with different fillet radii. The results show that the 20CrNiMo steel has high strength, high toughness and high plasticity. The stress concentration and strain concentration at notch root can improve the yield strength and tensile strength of the material, but reduce its plasticity. The material has a good resistance to notch embrittlement. The notch sensitivity coefficient is greater than 1, and it is not sensitive to notch. The macro fracture of smooth specimens is cup-cone shape, and fiber region, radiation region, shear lip can be seen. The micro fracture morphology is dimple shape, showing good plasticity. The cracking of notched samples originated from a certain distance from the bottom of the notched samples, and the periphery expanded in a “radial” pattern. The micro-section morphology showed dimple and quasi-cleavage composite fracture, and the notched samples suffered from brittle transgranular fracture under tensile stress.