Elementary school students have many difficulties with understanding of the particulate nature of matter. Many student difficulties were documented in different countries, and most of them are presented in the materials of the American Association for the Advancement of Science (AAAS), referred to as Project 2061. Fifteen problems were selected from that project, which had been pretested on a sample of 1000 American students from grade 6 -9, and which could be used to estimate Croatian students' comprehension of the particulate nature of matter. The testing was conducted in the elementary school O.Š. grofa Janka Draškovića in Zagreb on a sample of 109 students from grades 7 and 8. Results have shown that there was no statistical difference between students' success on the test in both grades. Some misconceptions proved to be resilient and appeared in both the 7th and 8th grade. In comparison to American students, Croatian students have shown significantly better results on the test. However, a large number of misconceptions that occurred with the American students were also found in Croatian elementary students. Finally, some suggestions for physics teaching are given which can help change students' misconceptions.