∵y=f(x)=|log0.5x|,∴f(1)=0,即1∈[a,b],由|log0.5x|=2得log0.5x=2或log0.5x=-2,解得x= 1 4 ,或x=4.∵y=|log0.5x|定义域为[a,b],值域为[0,2],∴当a= 1 4 时,1≤b≤4,当b=4时, 1 4 ≤a≤1,∴当a= 1 4 时,b=1时,区间长度最小为1- 1 4 = 3 4 .故答案为: 3 4 .