In coflow jets with the nozzle diameter of O (1 cm) and the fuel jet velocity of O (10 cm/s), the buoyancy induced by the density difference between the fuel and air influences the jet structure appreciably. The present study investigated the behavior of such a buoyant jet numerically and experimentally, especially when the fuel stream had higher density than air. When the fuel jet was composed of propane highly diluted with nitrogen, the fuel jet was decelerated and formed a stagnation region. Consequently, the fuel was carried downstream by the coflow having a circular cone shape. When the fuel was moderately diluted or as the jet velocity increased, numerical results showed the Kelvin-Helmholtz type instability along the mixing layer of the jet. When the fuel jet velocity was relatively high, the stagnation height increased nonlinearly with fuel jet velocity having the power of approximately 1.62. In the relatively high Reynolds number regime of Re > 80, the stagnation height can be correlated to $Re^{0.62}Ri^{-0.5}$, indicating the combined effects of buoyancy and jet momentum. As the Reynolds number becomes small, the stagnation height was affected by the streamwise diffusion due to fuel concentration gradient and by the wake behavior near the nozzle tip. Accordingly, the stagnation heights approach to none-zero values, which were found to be relatively insensitive to fuel dilution.