Job evaluation's main aim is to establish a fair wage structure. Its main principle is 'equal pay for equal work'. 'Metal Industry Job Grouping System' ( MIDS) has been in the metal industry in Turkey for more than 20 years. The results of the practice of the system were measured by the Gini coefficient. By the findings, the average wages of the job groups are different enough from each other by their job groups in the way the wage of the bigger number job group is higher than the smaller except the job group two in the industry. However, the wages are far from the principle 'equal wage for equal job' within the same job itself. In addition, the workers make use of bonuses and social benefits. The effect of social benefits of them on the wage structure is positive within the same job group and negative between different job groups within the same company. Nevertheless, the effect of both bonuses and social benefits on the wage structure is negative between the companies because, between them they are different from each other. Job group and seniority affect the wage structure. Seniority has a socio- psychological self- producing dynamic. The distribution of seniorities within the same job group has negative effects on the wage structure. However, this fact for each worker disappears for the long term since the worker of today who has short- term seniority will be the one of tomorrow who has long- term seniority. Therefore, for each worker, the wage structure gets closer to the principle 'equal wage for equal job' within the same job group itself over time.