The microwave dielectric properties and their related structural characteristics in solid solutions of (1-$chi$) Ba($Mg_{1/3}$Nb$_{2/3}$) $O_3$-$chi$La(Mg$_{2/3}$Nb$_{1}$3) $O_3$(BLMN) have been investigated by measuring the dielectric constant($varepsilon$r), Q value and temperature coefficient of resonant frequency($ au$f) and by observing the crystal structure using high resolution transmission electron microscopy (HRTEM). Microwave dielectric properties showed characteristic features for specific composition. Dielectric constant($varepsilon$$_{r}$) showed maximum value at the composition which corresponds to the phase boundary between 1 : 2 ordered and 1 : 1 ordered structure. The increase in $varepsilon$$_{r}$ may be caused by the rattling of ions by incorporating smaller ions and the disordered structure. The variation of temperature coefficient of resonant frequency($ au$$_{f}$) was investigated in terms of oxygen octahedra tilting.dra tilting.