The Signature Strengths are the most essential character strengths for a person's identity and uniqueness. The identification and utilization of one's signature strengths can promote mental health. The Character Strengths Test (CST) which is widely used for measuring signature strengths has excessive numbers of item that it may be inefficient in some situations, such as when there is not enough time or when it needs to be conducted with other measures. The purpose of this study is to conduct a preliminary investigation for developing the Signature Strengths Inventory (SSI), which is more efficient in identifying one's signature strengths. Signature Strengths Inventory, Character Strengths Test, Satisfaction with Life Scale, and CES-D were administered to 705 adult learners who attended online colleges in Seoul. Results showed that skewness and kurtosis of 24 character strengths scales met normality, and the internal consistency was adequate. The correlation analysis of the SSI with the CST showed that all of correlation coefficients were statistically significant. The rank-order correlation of the SSI with the CST was 74 and the concordance rate of top five character strengths was 80%, which means four out of five were identical. All of 24 character strengths by SSI showed significantly positive correlations with satisfaction in life and almost all of them showed significantly negative correlations with depression. Finally limitations of this study and suggestions for further research were discussed.