The proper connection number pc ( G ) of a connected graph G is defined as the minimum number of colors needed to color its edges, so that every pair of distinct vertices of G is connected by at least one path in G such that no two adjacent edges of the path are colored the same, and such a path is called a proper path. In this paper, we show that for every connected graph with diameter 2 and minimum degree at least 2, its proper connection number is at most 3 and the bound is sharp. Then, we give an upper bound 3 n ? + 1 - 1 of the proper connection number for every connected graph of order n ? 4 and minimum degree ?. We also show that for every connected graph G, the proper connection number pc ( G ) is upper bounded by pc ( G D ) + 2 , where D is a connected two-way (two-step) dominating set of G. Bounds of the form pc ( G ) ? 4 or pc ( G ) = 2 , for many special graph classes follow as easy corollaries from this result, which include connected interval graphs, asteroidal triple-free graphs, circular arc graphs, threshold graphs and chain graphs, all with minimum degree at least 2. Furthermore, we get the sharp upper bound 3 for the proper connection numbers of interval graphs and circular arc graphs through analyzing their structures.
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