Inkjet printing technology has been considered as the most promising process to fabricate full color Polymer Light Emitting Diode (PLED). However, ink droplet directionality of the print head significantly affects the yield rate for the manufacture of PLED displays during the printing processes. Because the physical properties of electro-luminescence polymer solution has a very large difference from the ink used in the commercial desktop printer. The driving signal of a commercial thermal bubble print head is not suitable for a print head designed for PLED. This study investigates how about the driving signal affecting the directionality of a print head for PLED solution's printing. With the specific optical observing system developed by ourselves, a fast and cost effective way to measure the directionality of a multi-nozzle print head become possible. The measuring processes include capturing the image of the ink drop on printing substrate firstly and then analyzing the images based on statistic and optimization theory. Three important parameters of the driving signal are discussed in this study. They are energy input, driving voltage and heating pulse width. The result gets an optimal combination of driving voltage and pulse width, which gives the best performance of droplet ejection directionality.