Happiness as the one of the most important human psychological needs, received remarkable attention, because of its marked impact on the human life. However, while there are many research evidence about negative affect, the evidence about happiness seems to be rare. The purpose of this study was to determine the relationship between vitality and demographic variables with happiness, in Isfahan population. Therefore, in the format of a correlation research design, 80 male and 80 female subjects were selected from Isfahan population, using cluster random sampling. All subjects responded to items of Oxford Happiness Inventory ( 0.82), Vitality Scale ( 0.79), and Demographic Questionnaire (including age, gender, marital status, education, academic performance, economic status, family size, income). The data were analyzed by Pearson's correlation coefficient, regression and analysis of covariance. The results showed that there was a significant positive correlation between happiness and vitality (P<0.01). There was also a significant negative correlation between happiness and family size (P<0.01), and no significant difference was found between the degree of happiness among male and female and also between single and married subjects. Therefore, it was concluded that the more vitality increases and family size decreases, the more happiness would be as a result.