Based on the attachment theory of psychology field, the study included three pre-tests and two experiments, and we chose 480 university students as our subjects. The study aimed to examine whether the co-brand evaluation was affected by the individuals with attachment style and the co-brand combination of two brand personalities. Moreover, we also investigated the relationships between the attachment of partner brand and co-brand evaluation and whether the product category fitness of partner brand and co-brand played a moderator role between both. The results were as follows: (1) Fearful attachment had a higher co-brand evaluation on the double-exciting personality brand than on the double-sincere personality brand. However, the preoccupied attachment had a lower cobrand evaluation on the double-exciting personality brand than on the double-sincere personality brand. (2) When the preoccupied attachment style was primed, the individuals had higher co-brand evaluation on the single personality (either double-sincere or double-exciting) than on the double personalities (both sincere and exciting). (3) The brand attachment of partner brand was positively related to the co-brand evaluation. Their relationship was moderated by the product category fitness. Our conclusions are helpful to the enterprises when they develop co-brand strategies for their targeted customers.