To understand how curvature affects the properties of stretched premixed flames including flame temperature, flame speed, and extinction, comparisons among the tubular premixed flame, the opposed jet flame and the one-dimensional planar flame are drawn physically, experimentally and numerically. By comparing the one-dimensional planar flame and the opposed jet flame, we give the physical process that produces the effects of stretch on laminar flame. The most important difference between them is that the former flow field is one-dimensional, i.e. convection is one-dimensional, and the latter has multi-dimensional convection. The interaction of flow divergence and heat and mass diffusion makes the temperature and flame speed of the stretched flame differ from one- dimensional planar flame; and the flow divergence ratio is proportional to Karlovitz number. By extending the analysis to tubular flame, we can see that the geometry of the tubular flame, i.e. curvature, strengthens the convection in axial direction according to one additional term, which is proportional to the square of Karlovitz number, in the expression of the flow divergence ratio. To prove this, numerical calculation for lean H2/air is presented here. Extinction experiments of lean H2/air, CH4/air and C3H8/air flames are also conducted to verify the analysis.