This study explored factors affecting self-disclosure and their changes among members in group counseling. Questionnaires were used to collect data from 58 clients in nine counseling groups each lasted for 10 sessions. Data from the second to the eighth sessions were gathered at the conclusion of each counseling session using the “Self-Disclosure Questionnaire(SDQ)”. Results indicated that: (1) at the initial phase (sessions 2 to 4),willingness to self-disclose is due to “Positive Relation” factor, and unwillingness to self –disclose is due to “uncertainty about self” and “self-deprecation” factors. (2) at the middle phase (sessions 5 to 7), willingness to self-disclose is related to similar factors as those in the initial phase, although the weights of “Positive Relationship” and “Break the Silence” factors slightly decreased; for unwillingness to self-disclose, the weights of the “Self Uncertainty” and “Self-Deprecation” factors slightly decreased, but the weights of “Personal Pace” and “Feeling Uncomfortable” slightly increased. (3) At the latter phase (sessions 8 to 10), willingness to self-disclose factors and their relative weights were about the same as that in the initial and middle phases; for unwillingness to self disclose, the factors of “Uncertainty” and “Self-Deprecation” factors were lower than that in the middle phase, and the factors of “Self-Deprecation,” “Lack of Trust,” “Personal Pace,” and “Feeling Uncomfortable” approached zero. Results were discussed and practical implications were suggested.