We have analyzed the drilling process with femtosecond laser on the silicon surface in order to investigate a degree of thermal effect during the dicing process of the very thin silicon substrate (thickness: 50 μm). Femtosecond laser pulse (E = 30~500 μJ/pulse, τ = 150~200 fs, λ = 780nm, f = 10 Hz) was focused on the silicon substrate using a lens with a focal length of 100 mm. An image-intensified CCD camera with a high-speed gate of 5 ns was utilized to take images of drilled hole during the processing. As a result, the rise time for increasing diameter of the holes was changed at energy between 180 and 350 μJ/pulse. The width of the molten walls around the hole became wider under the conditions where the rise time became longer. So, it is said that we can estimate the degree of the thermal effect qualitatively by analyzing the rise time. These knowledge is thought to be very important and useful for developing the dicing technique of thin silicon wafers by femtosecond lasers.