It is expected that more accounting conservation (environmental uncertainty) reduces manager optimism. Prior research, however, has struggled to establish this relation empirically. Moreover, some evidence points to the possibility that the manager optimism is lower for firms with more accounting conservation. In this paper, the author examine the link between accounting conservation, environmental uncertainty, and manager optimism, as well as the link between accounting conservation, manager optimism, and the cost of capital. First, it is established that more accounting conservation could lead to a decrease in manager optimism. Second, I tried to show that manager optimism changes the cost of capital in the accounting conservation quintile. Consistent with a negative relationship between accounting conservation and manager optimism, and with the dominating effect of managers' optimism, the article documented a positive association between managers' optimism and the cost of capital for firms whose shares trade in low accounting conservation.