The use of illustrations in poster designs is one of the creative and original ways of conveying the message effectively. The use of illustration in poster offers a way to express the message in an engaging and memorable way for the designer. When designing posters, many designers continue to use the benefits of illustration, applying the advantage of the new abilities provided by developing technologies. In many poster examples, various forms of expression have been set forth supported by the power of illustration. These forms of expression, strengthened by the use of illustration, have contributed to the language of graphic design in conveying the message. In this study, different expression forms of illustration in poster designs are reviewed and the findings are presented with descriptions and visual examples. The study includes examples of posters that can quickly describe the definitions and definitions obtained. A compilation on the classification of expressions through illustration is presented.