Background One child out of 5 is affected by a mental health problem before the age of 12 years. Among the determinants of children's mental health, Mental Health Literacy (MHL) is a theory including 6 dimensions: understanding mental health; help-seeking actions; supports available; influences on mental health; coping and resilience; and stigma. High levels of MHL correspond to better children's mental health. Measuring children's MHL is essential to inform the development of interventions pro-MHL, evaluate their impact and conceive related policies. We describe here the process of developing the CHILD-MHL scale. MethodsTwo public health researchers generate some MHL items in French. These are discussed by a group of 30 experts (ex. psychologists, psychiatrists), parents and teachers to verify their content and wording (Delphi study). An artist co-draw with 5 children (8-11 years old) the illustrations of the items (visual scale). Then, the scale is tested in the school setting by 30 children with feedback through qualitative interviews. Finally, the scale is administered to 500 children and undergoes psychometric analyses. Results Results are preliminary. We have produced 50 items telling a story based on MHL through the characters of Émile and Amélie, two children facing mental health problems. Examples of MHL items are: “Emile doesn’t say nice things about himself, he doesn’t want to spend time with his friend Amélie anymore, do you think he might be depressed?” and “Emile feels very worried and nervous, do you think he is anxious?”. Conclusions Developing a scale is a complex process. The CHILD-MHL scale is the first instrument measuring MHL in children in France and worldwide. It can be included in large French cohorts to obtain prevalence of MHL in representative samples of children at the national level. The scale will also be translated in 3 other languages (English, Spanish, Italian) with an international outreach and for cross-cultural comparison.< Réduire