Si3N4　ceramic　substrates　serving　as　heat　dissipater　and　supporting　component　are　required　to　have　excellent　thermal　and　mechanical　properties.　To　prepare　Si3N4　with　desirable　properties,　a　novel　two-step　gas-pressure　sintering　route　including　a　pre-sintering　step　followed　by　a　high-temperature　sintering　step　was　devised.　The　effects　of　pre-sintering　temperature　(1500　–　1600　°C)　on　the　phase　transformation,　microstructure,　thermal　and　mechanical　properties　of　the　samples　were　studied.　The　pre-sintering　temperature　played　an　important　role　in　adjusting　the　Si3N4　particles’　rearrangement　and　α→β　transformation　rate.　Furthermore,　the　densification　process　for　the　Si3N4　ceramics　prepared　via　the　two-step　gas-pressure　sintering　was　revealed.　After　sintered　at　1525　°C　for　3　h　followed　by　a　high-temperature　sintering　at　1850　°C　for　another　3　h,　the　prepared　Si3N4　compact　with　a　bimodal　microstructure　presented　the　highest　thermal　conductivity　and　flexural　strength　of　79.42　W·m−1·K−1　and　801　MPa,　respectively,　which　holds　great　application　prospects　as　ceramic　substrates.　©　2022　Elsevier　Ltd