In the present paper we present linear and weakly nonlinear models for the analysis of stability of particle-laden slightly curved shallow mixing layers. The corresponding linear stability problem is solved using spatial stability analysis. Growth rates of the most unstable mode are calculated for different values of the parameters of the problem. The accuracy of Gaster’s transformation away from the marginal stability curve is analyzed. Two weakly nonlinear methods are suggested in order to analyze the development of instability analytically above the threshold. One method uses parallel flow assumption. If a bed-friction number is slightly smaller than the critical value then it is shown that the evolution of the most unstable mode is governed by the complex Ginzburg-Landau equation. The second method assumes that the base flow is slightly changing downstream. Applying the WKB method we derive the first-order amplitude evolution equation for the amplitude.