Abstract: The comparison of differences in weldability between novel low manganese niobium microalloying steel (MnNb steel) and ordinary Q355 steel (Q355 steel) was performed deeply and systematically by methods of the highest hardness test in welding heat-affected zone, welding crack test in oblique Y-bevel, weldability evaluation test in typical butt joints and fatigue resistance in typical butt joints with both optical microscopy (OM) and scanning electron microscopy (SEM). The results revealed that under the condition of no preheating, there was the strong weld hardening tendency in Q355 steel, while MnNb steel had no tendency to weld hardening. Also, there were excellent properties of cold crack prevention of welding in both MnNb steel and Q355 steel with no preheating. Compared with Q355 steel joints, MnNb steel had slightly lower tensile properties and significantly better impact toughness, which is mainly due to the presence of fine intra-grain microstructure and ferrite side plate ferrite in coarse-grained heat affected zones (CGHAZs) of Q355 steel, while there were abundant acicular ferrite and small prior austenite grain in CGHAZs of MnNb steel. Besides, the conditional fatigue ultimate strength of MnNb steel joints was 57 MPa higher than that of Q355 steel joints. There was smaller space of fatigue striations in the fatigue expansion zone of MnNb steel joints indicating the slower crack expansion rate. Therefore, the fatigue performance of MnNb steel butt joints was better than that of Q355 steel butt joints significantly.