The dependency of conversational utterances on the mode of dialogue is analyzed. A speech corpus of 800 speakers collected under three different modes, i.e., talking to a human operator, an WOZ system and an ASR system, is used for analysis. Some characteristics such as sentence ;complexity loudness of the voice and speaking-rate are found to be significantly different among the dialogue modes. Linear regression analysis results also clarify the relative importance of those characteristics on speech recognition accuracy.