Abstract: In VLSI design,gate level fault simulation is often too slow to meet the demand of timeto-market.Thus the register transfer level（RTL) fault simulation has become a hot topic recently. Some RTL fault simulation methods have been proposed,but to get the total system fault coverage,the calculation of fault numbers or weighted coefficients need to synthesize RTL design into netlist.Based on signal sizes and operators,a method of RTL fault number prediction is proposed,and then a pure RTL fault coverage calculation method is developed in the paper.The experimental result proves the efficiency of the methods.