A fishnet based metamaterial (MTM) loaded rectangular microstrip patch antenna (RMPA) is proposed for THz applications. The negative refractive index of the MTM has been displayed at frequency range between 1.05 and 1.1 THz. This MTM can be used for improving the return loss, gain and radiation efficiency of RMPA. The return loss (S11), and gain are found as -57 and 3.57 dB, respectively. In addition, the bandwidth of -10 dB has been found as 8.2 %.