This study was conducted to determine the relationship between the by stanser effect with prosocial behavior in psychology students at the Medan Area University. Subjects in this study were 218 students of 2016 University of Medan Area Faculty of Psychology. The study employed purposive sampling technique. The data for bystander effect and prosocial behavior were collected by using two closed questionnaires. The results showed that there was a very significant and negative correlation between

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... stander effect with prosocial behavior (r=0,786; p= 0,000 < 0,05).Prosocial behavior in the psychology faculty at the University of Medan Area was classified as being shown a hypo thecal mean of 72.5> empirical mean of 59.49 where the difference between the two mean exceeds SD 13,029, the bystander effect was also classified as moderate because of hypothetical mean of 75> empirical mean of 63.49 where the second difference the mean exceeds the SD number 13,590. The coefficient of determination of the correlation r2 = 0.610 means that the bystander effect contributes effectively to prosocial behavior of 61.0% and from these results that there are still 38.2%. Other factors that influence in this study that are not visible include attractiveness, attribution for victims, there is a model, time pressure, the nature of victim's needs, mood ,nature, gender, place of residence and parenting