Estyn said by the end of primary school, many pupils in Wales have developed a good understanding of basic concepts such as gravity and magnetism.
Nearly all understand the importance of undertaking investigations carefully.
But the report said primary schools need to reduce the achievement gap between pupils eligible for free school meals and their classmates.
It added they should ensure the more able pupils also find science lessons challenging.
In February, Estyn said more able learners' progress was being stifled.
Estyn's report, looking at science and design and technology for seven to 11-year-olds, recommended schools make sure they know their strengths and weaknesses so they can improve.
It recommended local authorities and consortia should provide more training opportunities for teachers.
Primary schools need to make sure science lessons challenge all pupils, a report from the watchdog has said.