This study aimed at investigating different types of illustrations that are existing in science books from grade 8 to grade 10 in cycle two basic education schools in Oman and to determine their percentages in these books. This study tried also to know the different design criterias that are associated with these illustrations and the instructional functions that these illustrations serve in these books. To answer the questions of the study, the researcher has used two samples: the first sample consisted of the types of illustrations that were chosen from one unit from these books, and the second sample involved 300 male and female science teachers in cycle two basic education schools at the academic year -2009/2010 in Al-Batinah North and Al- Batinah South regions. To achieve the study aims, the researcher has used two instruments: 1. A list of Design criteria of illustrations that consists of (17) items. 2. A questionnaire that is directed to science teachers. It consists of (18) items which have been divided into four instructional functions which the illustrations serve. These functions are: the cognitive function, the affective function, the decorative function and the compensatory function. To enhance the reliability of the design list, the researcher used it to analyze the illustrations that were chosen from one unit of these books with the help of a specialist. Kaba coefficient between the two analysis was calculated and it was (0.96). To measure the reliability of the questionnaire, Cronbach alpha coefficient of internal consistency reached (0.93). This means that the two instruments were reliable to be used for the aim of this study. The study results revealed that: The percentages of different types of illustrations differed from one book to another, where the photographs and the diagrams got the highest percentages whereas the graphs, the cartoons and the maps got the lower ones. The design criteria of the illustrations existed in the books in different percentages, for example the percentage of the illustrations that are related to the objectives, concepts and to the text was (100%) and the percentage of the illustrations that are real and have a proper position in the books was also (100%). The other criteria got lower percentages. The mean of the instructional functions that were existing in grade 8 science book as evaluated by science teachers reached (2.67) and for science book of grade 9 was (2.64). This means that these functions are existing in these books with a middle degree whereas the mean of these functions in the grade 10 book was (2.55) which means a low degree for these functions in this book. According to the results of this study, it is recommended that to increase the number of the illustrations in these books for each concept as possible and to select the illustrations in accordance with standardized criteria and their functions. More research is also recommended on the same topic.