Fractional Sobolev space is a natural space coming from the trace results. It has three parameters namely. $0<s<1$, $p>1$, and $\Omega$ a bounded domain in $n$ dimensional Euclidean space. It was shown that for $sp \neq 1$ it can be embedded in weighted Lebesgue space with weight the distance from the boundary. It fails when $sp =1$. Here we deal with the critical case $sp=1$ and get the optimal embedding. This is a joint work with Prosenjit Roy, Purbita and Prosenjit Roy and Vivek Sahu.