Flexible microwave substrates have been fabricated using micron size powders of alpha-Al2O3 and MgO filled in polytetrafluoroethylene (PTFE) matrix through SMECH process, comprising of sigma mixing (SM), extrusion (E), calendering (C) followed by hot pressing (H). Prior to fabrication of the composites, silane coating on the fillers was done to preclude moisture absorption. Filler loading in the PTFE matrix was varied in steps (10-70wt.%) and their effects on the microwave dielectric properties (epsilon(r)' and tan delta) at X-band (8.2-12.4 GHz) were evaluated using waveguide cavity perturbation technique. Optimum filler loading for both alumina and magnesia filled composites were found out by systematically varying the filler concentration. SEM microgrpahs revealed uniform distribution of filler in the matrix without any delamintaion of stacked calendered tapes. A comparative study of the magnesia filled samples with and without silane coating is also carried out to ascertain the effects of brucite phase formed during silane coating process on the dielectric and moisture absorption properties of composite substrates. (C) 2008 Elsevier B.V. All rights reserved.