To reproduce: 
Add a RadTreeView to a form and use the following code:
private List<EntityA> entities = new List<EntityA>(); public Form1() { InitializeComponent(); radTreeView1.TriStateMode = true; entities = new List<EntityA>(); for (int i = 0; i < 10; i++) { var newEntityI = new EntityA(i.ToString()); for (int j = 0; j < 100; j++) { var newEntityJ = new EntityB(j.ToString()); for (int k = 0; k < 1000; k++) { var newEntityK = new EntityC(k.ToString()); newEntityJ.Entities.Add(newEntityK); } newEntityI.Entities.Add(newEntityJ); } entities.Add(newEntityI); } } private void checkBox1_CheckedChanged(object sender, EventArgs e) { foreach (var node in radTreeView1.Nodes) { node.Checked = checkBox1.Checked; } } private void radTreeView1_NodesNeeded(object sender, NodesNeededEventArgs e) { if (e.Parent == null) { foreach (var entity in entities) { e.Nodes.Add(new RadTreeNode(entity.Name, false) { CheckState = ToggleState.Off, Tag = entity }); } } else if (e.Parent.Tag is EntityA) { var currentEntity = e.Parent.Tag as EntityA; foreach (var entity in currentEntity.Entities) { e.Nodes.Add(new RadTreeNode(entity.Name, false) { CheckState = e.Parent.CheckState, Tag = entity }); } } else if (e.Parent.Tag is EntityB) { var currentEntity = e.Parent.Tag as EntityB; foreach (var entity in currentEntity.Entities) { e.Nodes.Add(new RadTreeNode(entity.Name, false) { CheckState = e.Parent.CheckState, Tag = entity }); } } else if (e.Parent.Tag is EntityC) { } } 
Put a breakpoint in the last If statement, you will notice that the breakpoint is being hit, while it is not if the TrieStateMode is set to false 

Workaround: 
Use the following custom RadTreeNode: 
public class MyTreeNode : RadTreeNode { private FieldInfo triStateField; public MyTreeNode() : base() { } public MyTreeNode(string text) : base(text) { } public MyTreeNode(string text, RadTreeNode[] children) : base(text, children) { } public MyTreeNode(string text, bool expanded) : base(text, expanded) { } public MyTreeNode(string text, Image image) : base(text, image) { } public MyTreeNode(string text, Image image, bool expanded) : base(text, image, expanded) { } public override ToggleState CheckState { get { return base.CheckState; } set { bool set = false; if (this.TreeViewElement != null && this.TreeViewElement.TriStateMode) { if (this.triStateField == null) { this.triStateField = typeof(RadTreeViewElement).GetField("triStateMode", BindingFlags.Instance | BindingFlags.NonPublic); } triStateField.SetValue(this.TreeViewElement, false); set = true; } base.CheckState = value; if (set) { this.triStateField.SetValue(this.TreeViewElement, true); } } } }