Iso-surface extraction is one of the most important approaches for volume rendering, and iso-contouring is one of the most effective methods for iso-surface extraction. Unlike most other methods having their search domain be the whole dataset, iso-contouring does its search only on a relatively small subset of the original dataset. This subset, called a seed-set, has the property that every iso-surface must intersect with it, and it could be built at the preprocessing time. When an iso-value is given at run time, an iso-contouring algorithm starts from the intersected cells in the seed-set, and gradually propagates to form the desired iso-surface(s). As a smaller seed-set could offer less cell searching time, most existing iso-contouring algorithms concentrate on how to identify a minimal seed-set. In this paper, we propose a new and efficient approach for the construction of a relatively small seed-set. This presented algorithm could reduce the size of a generated seed-set by up to one or two orders of magnitude, compared with other previously proposed linear-time algorithms.