Create an index group with the desired residues. This is a generic feature of just about any GROMACS analysis program. Or, in this simple case, you already have the values you want as a subset of the analysis already performed, so just parse those lines of interest from the output file.

ok I will try that ^_^

many thanks 

follow this (for residue 5 to 20)

Step 1.

make_ndx    -f   xx.tpr    -o  n.ndx

type

>    r    5-20     (Note- U ll see a new  index line number left to r_5_20; remember that;   let's say  21)

> q

Step 2.

g_rmsf    -s   xx.tpr    -f     xx.trr  -o  xxxxx.xvg  -n   n.ndx   -res 

now choose the number  21

Now plot  xxxxx.xvg