Dynamic Symbolic Execution (DSE) is among the most effective techniques for structural test generation, i.e., test generation to achieve high structural coverage. Despite its recent success, DSE still suffers from various problems such as the boundary problem when applied on various programs in practice. To assist problem diagnosis for structural test generation, in this paper, we propose a visualization approach named PexViz. Our approach helps the tool users better understand and diagnose the encountered problems by reducing the large search space for problem root causes by aggregating information gathered through DSE exploration.