Microwave dielectric properties of low-permittivity LiAl1−x(Mg0.5Si0.5)xO2 (LAMS) (x = 0.02–0.2) ceramics synthesised through solid-state reaction route were investigated. All compositions show pure LiAlO2 phase with a P41212 space group at x ≤ 0.18. Microstructure and relative permittivity (εr) were greatly affected by the partial substitution of [Mg0.5Si0.5]3+ for Al3+ site. Bond energy decreased with increasing x value, thereby increasing the temperature coefficient of resonant frequency (τf) in the negative direction. Finally, the optimum microwave dielectric properties of LAMS (x = 0.02) sample were obtained at 1300 °C with εr = 6.17, Q × f = 53,300 GHz and τf = − 129 ppm/°C. Then CaTiO3 was added to the LAMS (x = 0.02) material to adjust the τf value to near zero.