In triple patterning lithography (TPL), balanced feature density on each layout mask helps facilitate the following OPC process. This paper presents the first spacing uniformity-aware layout decomposition method, called SUALD, which formulates the density optimization problem in TPL based on the spacings between locally adjacent features on each colored layout mask, and hence enhances the patterning quality. Based on the new density formulation, a spacing uniformity graph is built using the Voronoi diagram. An effective heuristic triple partitioning algorithm is also proposed for TPL layout decomposition. Experimental results are very promising and show that SUALD obtains 69% and 40% improvements in average in the presented density metrics over an integer linear programming method without density control.