A novel design method to transform the radiation patterns of a narrow slot antenna under its first and second modes is proposed. Firstly, the far-zone radiated fields of the traditional slot antenna are deeply investigated at its first, second, and third modes. It demonstrates that the bidirectional radiation pattern is acquired for the first and third modes. By contrast, the second mode of the antenna commonly maintains the non-bidirectional radiation pattern. Secondly, the slot antenna is folded in order to transform its second mode into the bidirectional radiated fields. Thirdly, a linear slot is cut on the folded slot, aiming to reallocate its resonant frequency to the desired band. Finally, our proposed antenna is simulated and designed. The results show that the antenna is operated at around 2.50 GHz for the lower band and 4.23 GHz for the upper band, respectively. Most importantly, the second mode of the traditional slot antenna is successfully reshaped as the bidirectional pattern.