The effect of the heater geometry on the critical heat flux (CHF) in the saturated pool boiling condition was studied here using the vertically upward-facing thin flat plate heater mounted on a printed circuit board (PCB). The heater width was scaled by the characteristic width as the ratio of the width to the Laplace length, and the CHF data were correlated with the characteristic width. It was found that for the large characteristic width the present CHF data were almost independent of the heater width like the infinite flat plate heater. As the characteristic width of heater decreases, a large enhancement of CHF was observed. The CHF data measured were following the prediction of Lienhard and Dhir model with the exponent of -1/4 for the characteristic width. However, it was found that a lower limit exists in the application of the Lienhard and Dhir model. For the narrow heater with a characteristic width less than 0.25, larger enhancement of CHF was observed. We developed a new CHF correlation to explain this exceptionally large enhancement of CHF based on the existing Lienhard and Dhir model extending to a narrower region by introducing the wavelength as a function of the heater characteristic width. We determined this function experimentally from the wavelength measure from the air-water slit with the same width and length of the heater geometry and the distance of the bubble departure spots on the real heater surface. The present new model successfully estimated the CHF data observed with the exponent of -3/8 rather than that of -1/4 of Lienhard and Dhir. The present study partially explains the reason for the CHF enhancement of the heater with the specific micro- or nanostructure.