diff --git a/erpnext/projects/doctype/project/project.py b/erpnext/projects/doctype/project/project.py
index 2a8870b..df970f3 100644
--- a/erpnext/projects/doctype/project/project.py
+++ b/erpnext/projects/doctype/project/project.py
@@ -144,6 +144,9 @@
 		if self.sales_order:
 			frappe.db.set_value("Sales Order", self.sales_order, "project", self.name)
 
+	def on_trash(self):
+		frappe.db.set_value("Sales Order", {"project": self.name}, "project", "")
+
 	def update_percent_complete(self):
 		if self.percent_complete_method == "Manual":
 			if self.status == "Completed":
diff --git a/erpnext/projects/doctype/project/test_project.py b/erpnext/projects/doctype/project/test_project.py
index ebc1326..c64ac8d 100644
--- a/erpnext/projects/doctype/project/test_project.py
+++ b/erpnext/projects/doctype/project/test_project.py
@@ -9,6 +9,8 @@
 
 from erpnext.projects.doctype.project_template.test_project_template import make_project_template
 from erpnext.projects.doctype.task.test_task import create_task
+from erpnext.selling.doctype.sales_order.sales_order import make_project as make_project_from_so
+from erpnext.selling.doctype.sales_order.test_sales_order import make_sales_order
 
 test_records = frappe.get_test_records('Project')
 test_ignore = ["Sales Order"]
@@ -96,6 +98,21 @@
 
 		self.assertEqual(len(tasks), 2)
 
+	def test_project_linking_with_sales_order(self):
+		so = make_sales_order()
+		project = make_project_from_so(so.name)
+
+		project.save()
+		self.assertEqual(project.sales_order, so.name)
+
+		so.reload()
+		self.assertEqual(so.project, project.name)
+
+		project.delete()
+
+		so.reload()
+		self.assertFalse(so.project)
+
 def get_project(name, template):
 
 	project = frappe.get_doc(dict(