I. INTRODUCTIONA premixed flame is widely used for many combustion systems because it can produce high temperature from flames with low pollutant emissions. Regarding its wide application on domestic utilization in cooking stoves, multiple jets of the laminar premixed flames are produced for the heating processes. Even though the amount of gas used by each stove is small, more than 40 % of the total consumption of LPG (liquefied petroleum gas) in Thailand is used by this type of burner in the residential sector [1]. Thus, the improvement of thermal efficiency and emissions of cooking stoves operating with the premixed flame is a key target for present and future performance regulation. Many previous studies regarding single or multiple premixed flame jets have focused on the heat transfer process from flames to the target, i.e. flame impingement has been extensively studied. Both heat transfer characteristics (which is implied by heat flux distribution or local/averaged Nusselt number), and pollutant emissions resulting from impinging flame jets were experimentally studied as a function of different key parameters. These include Reynolds number, equivalence ratio, nozzle-to-plate distance, distance between the adjacent burners [2], and inclination angles [3]. These parameters influence flow pattern, which creates distinctly different flame shapes. Remie et al. [3] presented a simple relation that predicts the parameters of the burnt gas flow of a flame jet, Bunsen-type flame after expansion over the flame front. They stated that the parameters of the burnt gas flow, i.e. jet velocity and jet width, are important to estimate the heat transfer from flame jets to products. Thermal performance parameters and dynamic properties of the flow field at a flame impingement area, i.e. velocity magnitude, turbulent intensity, vorticity and strain rate, were reported [4]. These confirm the correlation between thermal efficiencies and flow fields which are controlling structures of the producing flame jets.Extensively, line-of-sight flame visualization techniques, e.g. direct photography of typical visible flame or imaging chemiluminescence emission of the combustion species , i.e. OH, CH and C2 were performed [5], [6]. However, the emission intensity was integrated along a viewing line. Different parts of the flame, such as the primary and the secondary combustion zones and the inter-zonal region, were observed. Radiation components were not separated. In-situ visualization technique was used to solidly clarify the real combustion situation in flames.This work investigates the variation of the main reaction zone as a function of the key parameters, i.e. primary equivalence ratio and inter-port spacing. Laminar premixed LPG-air multiple flame structure was observed using both typical line-of-sight observation and OH planar laser-induced fluorescence (OH PLIF) techniques. The OH PLIF was introduced to acquire the in-situ, two-dimensional image of the main reaction zone (i.e., regions containing high concentration of OH specie...