A group of new y M-phase/(1-y) Li2+xTi1-4xNb3xO3 composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li2+xTi1-4xNb3xO3 (x=0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li(2)TiO(3)ss" composition (x value). The best dielectric properties of epsilon(r)=40.1, Q x f values up to 9318 GHz, tau(f)=25 ppm/degrees C, were obtained for the ceramics composites at x=0.02. Based on the good dielectric properties, the suitable "Li(2)TiO(3)ss" composition with x=0.02 was mixed with the Li1.0Nb0.6Ti0.5O3 powder as the ratio of y "M-phase"/(1-y) "Li(2)TiO(3)ss" (y=0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from similar to 20 to similar to 60, while Q x f and tau(f) values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li(2)TiO(3)ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.