Sport-skill diagnosis is a complex process, which depends on the command of a large body of domain-specific knowledge that allows coaches to find solutions for technique deficiencies. Information from research on the diagnostic processes of coaches has the potential for practical applications to technique training and skill assessment. Additionally, it will help novice coaches and pre-service physical educators to develop instructional strategies and diagnostic ability of motor skill. The purpose of this study was to examine expert and novice volleyball coaches’ diagnostic ability of a volleyball skill. Four expert and four novice high school volleyball coaches participated in this study. Research procedures included an interview, a recall test, and the diagnostic task where the coaches analyzed the volleyball spike performance. Transcripts of both coaches’ verbal and nonverbal reports were sent to coaches for a member check, and used for data analysis. Results revealed that there were differences in diagnostic ability between expert and novice volleyball coaches. Expert coaches’ knowledge regarding the volleyball spike was richer and more extensive than that of novices in a) the number of the components identified, b) the number of body parts used to describe the skill, and c) in the judgment as to which issues are most critical in skill execution. Expert coaches’ explanations about an ideal spike demonstrated the knowledge that was more technically specific, and procedure-oriented, which may have been enhanced by their years of successful coaching experience. Expert coaches’ recall statements were more evaluative and technique-related. Although both expert and novice coaches reported a similar number of information cues during the skill diagnosis, experts perceived a greater number of technique deficiencies as compared to the novices. Findings from the present study also indicated the connections between coaches’ knowledge of the volleyball spike and their diagnostic ability. The study shows there is a need for novices to develop their motor skill diagnostic competency not only from classroom study, but also from field experience.