On the basis of the extended nonlinear propagation equation without normalization in metamaterials with the firstorder to thirdorder nonlinear dispersion effects, the dispersion relation and gain spectra of modulation instability were deduced by adopting the linear stability analysis and the Drude electromagnetic model. Variations of the equation coefficients and gain spectra with the normalized angular frequencies were calculated and discussed in real units in both the positive and negative refractiveindex regions in cases of selffocusing and selfdefocusing. The results show that, the thirdorder nonlinear dispersion coefficients are all along positive in both the positive and negative refractiveindex regions. In the negative refractiveindex region, with increase of normalized angular frequencies, the spectral widths of the gain spectra increase before become zero and the peak gains increase before decrease and then become zero in the selffocusing case. While in the case of selfdefocusing, modulation instability can only occur near the forbidden band and both the spectral widths and peak gains decrease monotonically. In the positive refractiveindex region, modulation instability can only occur in the selffocusing case. And the spectral widths increase monotonically and the peak gains decrease before increase with the normalized angular frequencies. Generally, the thirdorder nonlinear dispersion is detrimental to modulation instability generation.