A　series　of　numerical　simulations　were　performed　to　investigate　the　flame　radiant　heat　flux　of　horizontally　oriented　jet　fires　by　rectangular　source　fuel　with　the　same　orifice　area　(400　mm(2)　)　but　different　aspect　ratios　(1,　2,　4).　The　natural　gas　was　carried　out　as　fuel,　of　which　the　jet　velocity　varied　from　27.5　to　205.8　m/s.　The　nondimensional　radiant　heat　flux　was　defined　to　reveal　the　effect　of　fuel　jet　velocity　and　orifice　aspect　ratio　on　radiant　heat　flux.　Results　show　that　the　flame　size　increases　with　the　increasing　fuel　jet　velocity　and　the　larger　orifice　aspect　ratio　results　in　shorter　flame　length.　The　radiant　heat　flux　is　affected　by　the　orifice　aspect　ratio　as　the　different　soot　particle　distribution.　The　predicted　model　to　characterize　the　radiant　heat　flux　is　proposed　by　taking　the　orifice　aspect　ratio　into　account.　The　prediction　results　of　proposed　model　agree　well　with　the　simulated　data　and　previous　experimental　results.