Although researchers have argued that employees often carefully examine social contexts before speaking up to leaders, the role of leaders' affective states has received little attention. The current research addresses this important issue from an emotion-as-social-information perspective by exploring whether, why, and when leaders' affect influences employees' voice behavior. By collecting data of 640 daily interactions from both sides of 85 leader-employee dyads using the experience sampling method (ESM) through mobile surveys, we found that leaders' positive affect was positively related to employees' voice behavior. Furthermore, such a relationship could be accounted for through employees' psychological safety directly via emotional contagion mechanism (through employees' own positive affect) but not directly via signaling mechanism (through employees' assessment of leaders' positive affect), and the effects of both employees' own positive affect and their assessments of leaders' positive affect on psychological safety were stronger when the leader-member exchange relationship was weak. Interestingly, we also found that leaders' negative affect was positively related to employees' voice, but neither emotional contagion nor signaling mechanisms explained this effect. These findings highlight the important role of leaders' affect in the voice process and also provide insights for when employees would choose to speak up to their leaders.