Flame stability and mean structure of partially premixed flames have been investigated under the effect of the level of partial premixing and nozzle cone angle. The stability curves and maps of the mean flame structure based on temperature and CO and O-2 concentrations measurements in some selected partially premixed flames in the thin reaction zones regime are presented and discussed. More radial and axial mean profiles of temperature and CO and O-2 concentrations are also presented for another set of flames at the same equivalence ratio and several nozzle cone angles. The data show that partially premixed flames are more stable than non-premixed and premixed flames. An optimum degree of partial premixing was achieved in the present burner, beyond which the flames are less stable. This optimum level was achieved when the dimensionless mixing length normalized by the nozzle diameter is equal to 5. At this level of partial premixing the structure is likely to form three interacting reaction zones of lean, rich and diffusion with expected triple flame structure. In partially premixed flames a stabilization core has been observed close to the conical nozzle that provides more heat source at the nozzle exit. This is responsible for stabilizing the flames at high Reynolds number. The data also show that the cone angle has a great influence on the flame stability. Increasing the cone angle leads to more air entrainment, breaking the stabilization core and hence reduces the flame stability. The cone, in all cases, provides protected environment at the early stage of reaction near the nozzle exit where intense turbulence is expected. This leads to highly stable flames as compared to similar burners without cone.