thanks jeff, find missing lines in file data structure , fill missing lines. however, filled missing lines in output show "2013-07-01 00:00:00,,,,,,,,,,,,,,,,,,", not nan. fill "nan" or "nan" inside of commas. my code is: filin = datapath + 'skp_aws_min_qc_10001_2013.07-09.dat' pd.set_option('max_rows',10) data=pd.read_csv(filin,sep='#',index_col=[1],parse_dates=[1]) print data index = pd.date_range('2013-07-01 00:00:00','2013-09-30 23:59:00',freq="t") df = data sk_f = df.reindex(index) print sk_f sk_f.to_csv("sample1.csv") as long know information reindex function, missing holes should filled (default "nan"). can't find reason why missing hole in result files not filled. any idea or comment appreciated. thank you, isaac it has nothing reindex() , when to_csv , provide string missing values if don't want them show blank spaces. df.to_csv('temp.csv'