diff --git a/app/api/routers/projects.py b/app/api/routers/projects.py
index 0424275..d790144 100644
--- a/app/api/routers/projects.py
+++ b/app/api/routers/projects.py
@@ -228,6 +228,38 @@ def delete_project(
     project = db.query(models.Project).filter(models.Project.id == project_id).first()
     if not project:
         raise HTTPException(status_code=404, detail="Project not found")
+    
+    project_code = project.project_code
+    
+    # Delete milestones and their issues
+    from app.models.milestone import Milestone
+    milestones = db.query(Milestone).filter(Milestone.project_id == project_id).all()
+    for ms in milestones:
+        # Delete issues under milestone
+        issues = db.query(models.Issue).filter(models.Issue.milestone_id == ms.id).all()
+        for issue in issues:
+            db.delete(issue)
+        db.delete(ms)
+    
+    # Delete project members
+    members = db.query(models.ProjectMember).filter(models.ProjectMember.project_id == project_id).all()
+    for m in members:
+        db.delete(m)
+    
+    # Remove from other projects' sub_projects and related_projects
+    import json
+    all_projects = db.query(models.Project).all()
+    for p in all_projects:
+        if p.sub_projects and project_code in p.sub_projects:
+            subs = json.loads(p.sub_projects) if p.sub_projects else []
+            subs = [s for s in subs if s != project_code]
+            p.sub_projects = json.dumps(subs) if subs else None
+        
+        if p.related_projects and project_code in p.related_projects:
+            related = json.loads(p.related_projects) if p.related_projects else []
+            related = [r for r in related if r != project_code]
+            p.related_projects = json.dumps(related) if related else None
+    
     db.delete(project)
     db.commit()
     return None