CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V1/2 (bias voltage at half maximum TMR ratio) and Vout (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (Ta) and Ru thickness (tRu) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V1/2 of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V1/2 for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with tRu= 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. Vout in the positive and negative voltage branches follows the same trend as that of TMR ratio with Ta.