In this paper, the study of effects of flow parameters on flame structure and NOx emission concentration was performed in co-axial. laminar partially premixed methane/air flames. Such (low parameters as equivalence ratio(${Phi}$), fuel split percentage($sigma$), and mixing distance(x/D$\_$i/) were defined as a premixing degree and varied within ${Phi}$=1.36∼9.52, $sigma$=50∼100, and x/D$\_$i/=5∼20. The image of OH$^$*/ and NOx concentration were obtained with an ICCD camera and a NOx analyzer. The flame structure observations show a categorization of partially premixed flames into three distinct flame regimes corresponding to ${Phi}$<1.7(premixed flame structure), 1.7<${Phi}$<3.3(hybrid structure), and ${Phi}$>3.3(diffusion flame structure existing a luminous sooting region) at $sigma$=75%, and x/D$\_$i/=10. As o decreases from 100% to 50%, and x/D$\_$i/ decreases, nonpremixed flame structure appear at low equivalence ratio relatively. In addition, the measured emissions for NOx rise steeply from ${Phi}$=1.7, to ${Phi}$=3.3, then constants ${Phi}$>4.76. NOx emissions decrease with increase the level of premixing level. In conclusion, the main effect on flame structure and NOx production was at first equivalence ratio(${Phi}$), and next fuel split percentage($sigma$), and finally mixing distance(x/D$\_$i/).