Preparing relevant and useful information for making decision is the aim of the use of cost system. The cost system can affect information relevance and usefulness. This article is to investigate the relation between cost system function and managers evaluation of the relevance and usefulness of cost data. Given the importance, extent, and complexity of petrochemical industry, this paper uses petrochemical industry as statistical population. Based on the data of sixty questionnaires collected, research hypotheses are tested by accessing the significance of Ordinary Least Square coefficients. The cost system function divided into four categories: information details, information classification, reporting frequency, and variances reports. The results of first hypothesis confirms that cost system function affects manager’s evaluations of the usefulness of cost data. This confirmation implies that regression coefficients were significant and positive for information details, information classification, and variances reports. But, the second hypothesis that investigates the effects of cost system function on the relevance of information is rejected.