diff --git a/rabit-learn/solver/lbfgs.h b/rabit-learn/solver/lbfgs.h
index 36ce4d25d..8fe728103 100644
--- a/rabit-learn/solver/lbfgs.h
+++ b/rabit-learn/solver/lbfgs.h
@@ -177,7 +177,7 @@ class LBFGSSolver {
     // swap new weight 
     std::swap(g.weight, g.grad);
     // check stop condition
-    if (gstate.num_iteration > min_lbfgs_iter) {
+    if (gstate.num_iteration > static_cast<int>(min_lbfgs_iter)) {
       if (g.old_objval - g.new_objval < lbfgs_stop_tol * g.init_objval) {
         return true;
       }
@@ -196,7 +196,7 @@ class LBFGSSolver {
   /*! \brief run optimization */
   virtual void Run(void) {
     this->Init();
-    while (gstate.num_iteration < max_lbfgs_iter) {
+    while (gstate.num_iteration < static_cast<size_t>(max_lbfgs_iter)) {
       if (this->UpdateOneIter()) break;
     }
     if (silent == 0 && rabit::GetRank() == 0) {