Knobe (2003) demonstrated that people’s intentionality judgments of side-effects de- pend on whether the consequence is positive or negative. This indicates that people’s judgments of intentionality of action depend not only on their perception of the inten- tion of the actor but also on the results of the action. The current study examines the Knobe effect in terms of causal structure and probability. To address these issues, this study employed almost the same experimental procedure as Knobe’s original experi- ment (2003). We also added a condition where participants were required to consider the intentionality of an action whose side effect also affected the actor. In addition, this condition required intentionality and probability judgments about outcomes. The results demonstrated that both causal structure and probability play an important role in the Knobe effect.