Modeling　and　analyzing　architectural　vulnerability　factor　(AVF)　of　microprocessors　is　helpful　in　reliability-aware　microarchitecture　design.　This　paper　derives　formulas　for　computing　AVF　of　critical　pipeline　structures　by　calculating　the　instruction　occupancy　of　a　structure　based　on　instruction-level　parallelism.　A　mechanistic　relationship　is　shown　to　exist　between　instruction　fetch　width　and　basic　block　size　to　AVF.　There　is　an　upper　bound　threshold　impact　of　variable　fetch　width　and　basic　block　size　on　AVF,　and　the　value　of　this　threshold　can　be　calculated.　There　is　a　quadratic　relationship　for　AVF　from　the　minimum　value　between　fetch　width　and　basic　block　size.　Experimental　results　prove　the　above　conclusion.　Based　on　this　mechanistic　relationship,　a　design　architect　can　rapidly　evaluate　reliability,　choose　corresponding　parameters　at　an　early　design　stage,　and　obtain　a　better　trade-off　between　reliability,　performance,　and　cost.