Grass-based dairy farming systems located in mountain areas and complying with Protected Designation of Origin specifications provide multiple services to these territories. However, the number of dairy farms has decreased since the 80s. Consequently, there is a need to assess the sustainability of these farms. As on-farm working conditions are a major issue of the sustainability of farms, this study aims to develop a tool to assess on-farm working condition (FWC) on a farm scale to detect possible leeway to improve sustainability of dairy farms and to maintain milk production in the mountains. To design a relevant and meaningful tool for the target users –the farmers– relying on a pertinent and exhaustive overview of working conditions of the PDO dairy systems, a participatory approach has been chosen. First, a focus-group, including farmers, advisors and researchers, defined criteria covering the different aspects of working conditions. Then, they selected or created appropriate indicators out of a list of possible existing indicators. Afterwards, the index was built by weighting indicators and criteria. This process results in an index made up of 5 criteria described by 40 indicators with original criteria compared to former index such as mental dimension of work. The index will be further tested on dairy farms. This index, designed by and for the farmers will be implemented on a large number of PDO dairy farms to obtain more insight on their FWC and to help advisors to support farmers to improve their FWC and their sustainability.