Process tracing approach was adopted to explore the personal decision behavior under choice situation, with Beach & Mitchell (1978)’s cost-benefit model as the framework. Namely, the purposes of this study were twofold: (1) To investigate the effects of task complexity on personal information processing, i.e., to analyze the interaction effects of time pressure, the number of attributes, and the number of alternatives on the depth and the sequence of searches. (2) To explore the reationship between task complexity and the patterns of decision strategy. Forty-five undergraduates served as subjects. A 3 (Time Pressure)×2 (Number of Attributes)×2 (Number of Alternatives) mixed design was used, with the second and the third as within-subjects factors. The dependent variables were the amount of information used, Payne Index, variability of search (SD), and the frequency of each decision strategy. The findings were as follows: (1) Both the number of attributes and the number of alternatives had effects of the depth of search. As the number of attributes or the number of alternatives increased, subjects searched less amount of information. (2) There was significant interaction effect between the number of attributes and the number of alternatives, when Payne Index was used. In other words, when subjects were making 4×8 choice task, information search was dominated by intra-dimensional search; however, the same information search way was not found when making 4×4 or 8×8 choice task. On the other hand, the variability of search increased when eight attributes or eight alternatives were in the choice set. (3) The search used was related to the number of attributes and to the number of alternatives. When the number of attributes or the number of alternatives increased, subjects used many noncompensatory strategies of which the conjunctive strategy was the dominant mode. Finally, theoretical implications for effects task complexity on decision behavior were discussed. The suggestions for the terms of research method were also discussed.