As environmental standards are not harmonized in ports, neither are the measures for evaluating the sustainability of port operations, nor the instruments for the assessment of the current condition of environmental efficiency of a port system. The purpose of the research conducted in the framework of the thesis is to define the environmental index as a value which reflects the level of influence of port processes on the environment. The goals of the thesis were the following: (1) to integrate relevant indicators of port activities and evaluate the influence of port terminal processes on the environment through the formation of an environmental index and (2) to validate the environmental index through the elaboration of a case study on the container terminal Brajdica. The environmental index will be construed by means of the compound index method, whereby certain statistical methods, like compilation and aggregation of more individual indicators, are used to form a compound index. By means of the obtained index, it will be possible to evaluate the overall effect of port processes on the environment. The validation of the environmental index through the application of the index on the Port of Rijeka terminal showed that the index can be easily applied, and its application enables us to follow the changes in environmental performance of port areas. The environmental index calculated by means of the suggested model may be used to show the progress in port terminal operation in the context of pollution reduction in port area. Furthermore, the obtained index can be used to evaluate the current effect of a port system on the environment, but also to identify trends of a port system implying its sustainability. Also, the application of this methodology may be used to test certain interventions in the port processes and determine their share in the port terminal influence on the environment. In addition, the paper considers the potential restraints of such approach.