Pulsed laser deposition (PLD) technique has been widely used in thin film preparation because of its wonderful and excellent properties. Boron carbide (B4C) thin films are recognized to have potential for applications like hard coating and electron field emission devices. B4C is the third hardest material after diamond and cBN, with a highest hardness of HV similar to5000. Furthermore, B4C is interesting from the point of view of wear resistance and stability at high temperature. We have deposited B4C thin films by KrF excimer laser (lambda = 248 nm) ablation of a stoichiometric B4C target in high vacuum. In this paper, we have prepared the B4C thin films on Si(100) substrates under various conditions, such as substrate negative DC bias voltage from between 0 to 800 V, substrate temperatures from room temperature up to 550 degreesC, and laser fluence between 2 and 8 J/cm(2) on the target. The typical absorption of B-C stretching bonds was detected from the infrared absorption measurement by FTIR of the deposited B4C thin films. We concluded that the hardness of the B4C thin films increases as the boron/carbon (B/C) ratio increases. The B/C ratio depends on the laser fluence. We obtained the maximum B/C ratio of 3.42 at 5 J/cm(2). The hardness of that sample reached 5.84 times of that of the Si(100) substrate.