It is a widely-discussed important issue to determine whether generation capacity adequacy could be ensured by the energy-only market approach. First， a theoretical investigation is made for this market mode under perfect competition assumption， and it is concluded that there is no enough theoretical evidence that the energy-only market approach could ensure the generation capacity adequacy. Secondly， actual operation performance of those electricity markets employing this market mode is summarized， and it is found that there is some difficulty for this market mode to induce enough generation investment. Both the theoretical results under perfect competition assumption and the operating performance in an actual electricity market environment illustrate that it is difficult for the energy-only market to ensure generation capacity adequacy. The analysis is helpful to the comprehensive understanding of the features of the generation capacity adequacy problem.