ObjectiveTo investigate the lateralization of ictal scalp EEG in different times in focal epilepsy.Methods356 surface ictal EEG of 41 patients were reviewed retrospectively in focal epilepsy arising from the mesial frontal, lateralfrontal, mesialtemporal, neocorticaltemporal, insular lobes and posterior cortex from July, 2010 to at, 2016. Each ictal scalp EEG was subdivided into ten epoches (E1-E10), then the lateralization of every epoch was analyzed. Ten epochs EEG were merged into three timesas E1-E3, E4-E6 and E7-E10. The ratio of lateralization, mislateralization and non-lateralization of each timeEEG were studied. Ictal onset zone (IOZ) were precise localized by intracranial EEG. The results of epileptogenic zone corresponded with surgical outcomes as seizure free or decreased.Results62% seizures were lateralized by surface ictal EEG in all epilepsies. Lateralized ictal scalp EEG were seen in nearly 80% of seizures in all times in temporal lobe epilepsy (TLE). The highest lateralization of 89% occurred inE4-E6 andfalse lateralization up to 30% in E1-E3 in mesial temporal lobe epilepsy (MTLE), whereas 95% lateralized seizures emerged in E1-E3 in neocortical temporal lobe epilepsy (NTLE). Apparent non-lateralization in all times were higher than lateralization in frontal lobe epilepsy (FLE), especially in mesial frontal lobe epilepsy (MFLE). Lateralization in E1-E3 was only 24% higher than other times. In addition, False lateralization never occurred in all times in lateral frontal lobe epilepsy (LFLE). There were maximum of 83%lateralized seizures in E1-E3 in LFLE and 93% in E1-E3 in posterior cortex epilepsy (PCE). Seizures arising from insular lobe epilepsy (ILE) tendedto predict less lateralization in all times.ConclusionsIctal scalp EEG of E1-E3 are valuable in the lateralization in all epilepsies particularly in LFLE, NTLE and PCE. Lateralized E4-E6 and E7-10 are very useful in MTLE.