A biological circuit design tool composed of Auto Mode and Manual Mode. We classified the parts released in 2013 and constructed a database for users to choose what they want. We use AHP (Analytic Hierarchy Process) to score parts and edges (passage linking two parts) according to parts quality including availability, usefulness, sample status, part status and sequencing. According to weight of edges, we present “Index of stability” to users. Users can save the circuits designed in Nebula in case they want to check or change it later.