In this study, the intergranular insulating Fe-6.5wt.%Si/SiO2 soft magnetic composites (SMCs) were prepared successfully using in-situ chemical deposition followed by the spark plasma sintering (SPS) process. The effects of ammonia concentration on the microstructure and magnetic properties of the composites have been studied systematically. The Fe-6.5wt.%Si alloy particles could be well insulated by the uniform SiO2 insulating layer, and its thickness increases with increasing the ammonia concentration from 0 to 0.02 ml/g. However, further increasing the ammonia concentration to 0.03 and 0.04 ml/g would result in the discontinuous and uneven SiO2 insulating layer. Correspondingly, the saturation magnetization and effective permeability of the composite compacts first decrease and then increase with increasing the ammonia concentration from 0.00 to 0.04 ml/g, whereas the coercivity and resistivity vary in the opposite tendency. Note that the overall performances such as the frequency stability of effective permeability, higher resistivity and lower total core loss, reach the optimal value for the sample with the ammonia concentration of 0.02 ml/g. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.