It is well known that in spoken conversation, unlike in written text, noun phrases often appear without being marked by a particle in Japanese. While this phenomenon has mainly been treated in terms of 'particle omission' in previous research, there has been a new approach to the phenomenon, which assumes that the phenomenon is a result of the manifestation of the 'zero particle', a grammatically independent entity without phonetic realization. The current study adopts the initial spirit of the new approach, and aims to explore the nature of the zero particle, and to shed light on some issues of spoken conversation. Through the study, it will be demonstrated that the zero particle has two main properties. One is a grammatical function ('absolute specification'), by which the speaker specifies an object or event without referring to other objects/events. The other is an expressive effect ('compensatory reinforcement' of interactive mood), by which the expression becomes indicative of stronger and more straightforward feelings and emotions. In connection with these two properties, the study addresses the relevant issues, including the motivation of the frequent use of the zero particle in spoken conversation, and the relationship between the zero particle and formality.