The mechanical properties and work hardening behavior of columnar-grained HAl77-2 brass were investigated by means of room temperature tensile test, EBSD and TEM. The effects of grain size on the work hardening rate and tensile ductility of the alloy were discussed. Some references reported that deformation twinning developed in the equiaxed-grained brass led to a reduction in slip length of dislocation and an increase in the work hardening rate at the second stage in the curve of work-hardening vs strain. In this work, however, the results showed that the low-angle subgrain boundaries distributed parallelly were formed in the columnar grain and reduced the slip length of dislocation at the second stage, which was responsible for the rise of the work hardening rate. With increasing grain size, both the yield strength and ultimate tensile strength of the columnar-grained HAl77-2 brass decreased, but its elongation to failure increased significantly from 70.4% for the grain size of 2.0 mm to 84.4% for the grain size of 6.0 mm. Higher performance to resist the plastic instability and better deformation uniformity mainly contributed to the ductility improvement of the larger-grain-sized columnar-grained HAl77-2 brass.