The millimeter wave (mmWave) band has gained significant attention due to its potential to cater to the rapidly increasing wireless data rates. Due to the reduced wavelength in mmWave communications, it is possible to implement large antenna arrays at both the transmitter and the receiver. Designing small antennas in the mmWave range presents many challenges, which is the main aim of this paper. The aim of this work is to proposed an efficient design of a dual-band mmWave antenna, with the dimension of 26.5mm×7.0mm×0.254mm, for future cellular communication systems using a substrate integrated waveguide (SIW). The elements of the proposed antenna consist of SIW cavity with one longer longitudinal slot and another shorter engraved slot in one of the conducting planes (1×2) for 28 GHz and 38 GHz, respectively. The substrate duroid 5880/Rogers are used with a loss tangent and dielectric constant of 0.003 and 2.2, respectively. The CST Microwave Studio, an industry-standard software, was utilized to conduct the simulation results. The proposed antenna's performance was evaluated by analyzing its gain, radiation pattern, and return loss at the frequencies of 28 GHz and 38 GHz. Furthermore, it is compared with other relative works. The single antenna element was able to attain an impedance bandwidth (S11< -10 dB) of 1.32 GHz and 3.1 GHz, with a satisfactory gain of 6.1 dBi and 5.81 dBi at 28 GHz and 38 GHz, respectively. The results indicate that the designed antenna can attain consistent and adjustable dual-frequency performance, making it a viable option for future cellular communication systems.