Experiments are conducted to elucidate effects of counterflow burner diameter on flame extinction behaviors in C-curve. Present experimental results with burner diameters of 18, 26, and 50 mm in normal-gravity are compared with the numerical result of Oppdif code as well as the previous experimental results in micro-gravity. The turning point migrates to a higher global strain rate as burner diameter decreases. It is shown that the C-curve with the burner diameter of 50mm is best-fitted to the numerical result of Oppdif code and the previous micro-gravity results also excurse to the numerical result. This suggests that the precise C-curve can be obtained only with an appreciably large burner. The main reason why these differences appear is shown to be attributed to the transition of shrinking flame disk to flame hole due to strong effects of radial conduction heat loss, which is the typical extinction characteristics of low strain rate flames with a finite burner diameter in a counterflow diffusion flame.