diff --git a/tacker/tests/unit/vnfm/test_plugin.py b/tacker/tests/unit/vnfm/test_plugin.py
index be03bbf1f..fef337ef0 100644
--- a/tacker/tests/unit/vnfm/test_plugin.py
+++ b/tacker/tests/unit/vnfm/test_plugin.py
@@ -29,6 +29,7 @@ from tacker.extensions import vnfm
 from tacker.plugins.common import constants
 from tacker.tests.unit.db import base as db_base
 from tacker.tests.unit.db import utils
+from tacker.vnfm import monitor
 from tacker.vnfm import plugin
 
 
@@ -110,6 +111,12 @@ class TestVNFMPluginMonitor(db_base.SqlTestCase):
         }]
 
         mock_get_vnfs.return_value = vnfs
+        # NOTE(bhagyashris): VNFMonitor class is using a singleton pattern
+        # and '_hosting_vnfs' is defined as a class level attribute.
+        # If one of the unit test adds a VNF to monitor it will show up here
+        # provided both the unit tests runs in the same process.
+        # Hence, you must reinitialize '_hosting_vnfs' to empty dict.
+        monitor.VNFMonitor._hosting_vnfs = dict()
         vnfm_plugin = plugin.VNFMPlugin()
         hosting_vnfs = vnfm_plugin._vnf_monitor._hosting_vnfs.values()
         hosting_vnf = hosting_vnfs[0]['vnf']
diff --git a/tacker/vnfm/monitor.py b/tacker/vnfm/monitor.py
index b8aa3dbc3..ad45c1f09 100644
--- a/tacker/vnfm/monitor.py
+++ b/tacker/vnfm/monitor.py
@@ -95,7 +95,7 @@ class VNFMonitor(object):
             time.sleep(self._status_check_intvl)
 
             with self._lock:
-                for hosting_vnf in self._hosting_vnfs.values():
+                for hosting_vnf in VNFMonitor._hosting_vnfs.values():
                     if hosting_vnf.get('dead', False):
                         LOG.debug('monitor skips dead vnf %s', hosting_vnf)
                         continue
@@ -120,7 +120,7 @@ class VNFMonitor(object):
                    'ips': new_vnf['management_ip_addresses']})
         new_vnf['boot_at'] = timeutils.utcnow()
         with self._lock:
-            self._hosting_vnfs[new_vnf['id']] = new_vnf
+            VNFMonitor._hosting_vnfs[new_vnf['id']] = new_vnf
 
         attrib_dict = new_vnf['vnf']['attributes']
         mon_policy_dict = attrib_dict['monitoring_policy']
@@ -132,7 +132,7 @@ class VNFMonitor(object):
     def delete_hosting_vnf(self, vnf_id):
         LOG.debug('deleting vnf_id %(vnf_id)s', {'vnf_id': vnf_id})
         with self._lock:
-            hosting_vnf = self._hosting_vnfs.pop(vnf_id, None)
+            hosting_vnf = VNFMonitor._hosting_vnfs.pop(vnf_id, None)
             if hosting_vnf:
                 LOG.debug('deleting vnf_id %(vnf_id)s, Mgmt IP %(ips)s',
                           {'vnf_id': vnf_id,
@@ -175,7 +175,7 @@ class VNFMonitor(object):
                     hosting_vnf['action_cb'](action)
 
     def mark_dead(self, vnf_id):
-        self._hosting_vnfs[vnf_id]['dead'] = True
+        VNFMonitor._hosting_vnfs[vnf_id]['dead'] = True
 
     def _invoke(self, driver, **kwargs):
         method = inspect.stack()[1][3]
diff --git a/tox.ini b/tox.ini
index c1ab409cf..3689687df 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,7 +17,7 @@ install_command =
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
-   stestr run --slowest {posargs}
+   stestr run --slowest --concurrency 1 {posargs}
 
 [testenv:functional]
 setenv = {[testenv]setenv}