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To protect the environment and enhance the human health, the lead-free process is speeded up rapidly in recent years. However, on the way to lead-free process, lead and lead-free materials will be used within the same mounting process for the manufacturers of electronic products. Therefore it is necessary to study the reliability of the mixed solder joints. Based on BGA device, this paper analyzes the reliability of the mixed solder joints with thermal cycling loading by ANSYS software, and predicts the thermal fatigue life of the joints by modified Coffin-Manson equation. The order of the thermal fatigue life of the joints is: lead-free solder joints >FWC solder joints >BWC solder joints (uniform structure) >lead solder joints >BWC solder joints (non-uniform structure). A case study is given. The result shows that the reliability of lead-free solder joints and mixed solder joints (uniform structure) are better than that of lead solder joints.