This paper discusses the cognitive differences between reasoning with Euler diagrams and reasoning with Venn diagrams. We test subjects' performances in syllogism solving in case where these two types of diagrams are used. We conduct an analysis on the role played by the conventional devices of each diagram in reasoning processes. Based on this, we hypothesize that of the two types of diagrams, only Euler diagrams could guide subjects without prior knowledge of their inferential strategies for combining diagrams. To test this hypothesis, subjects in our experiment are only provided with instructions on the meanings of diagrams and required to solve reasoning tasks without any instruction on the solving strategies. Our experimental results support the hypothesis and indicate that Euler diagrams can not only contribute to subjects' correct interpretation of categorical sentences used but also play a crucial role in reasoning processes themselves.