After great trial, I succeeded to solve it with the following way..
SELECT ptnt_vst_csno
FROM table_name
ORDER BY Substring(ptnt_vst_csno, 1, Charindex('P', ptnt_vst_csno)),
CONVERT(INT, Substring(Substring(ptnt_vst_csno,
Charindex('P', ptnt_vst_csno),
Len(
ptnt_vst_csno)), 2, Len(
ptnt_vst_csno)))