diff --git a/R-package/tests/testthat/test_basic.R b/R-package/tests/testthat/test_basic.R
index 3cbf7f991884..2f2f9efc32d3 100644
--- a/R-package/tests/testthat/test_basic.R
+++ b/R-package/tests/testthat/test_basic.R
@@ -245,11 +245,12 @@ test_that("training continuation works", {
     expect_equal(bst$raw, bst2$raw)
   expect_equal(dim(bst2$evaluation_log), c(2, 2))
   # test continuing from a model in file
-  xgb.save(bst1, "xgboost.model")
-  bst2 <- xgb.train(param, dtrain, nrounds = 2, watchlist, verbose = 0, xgb_model = "xgboost.model")
+  xgb.save(bst1, "xgboost.json")
+  bst2 <- xgb.train(param, dtrain, nrounds = 2, watchlist, verbose = 0, xgb_model = "xgboost.json")
   if (!windows_flag && !solaris_flag)
     expect_equal(bst$raw, bst2$raw)
   expect_equal(dim(bst2$evaluation_log), c(2, 2))
+  file.remove("xgboost.json")
 })
 
 test_that("model serialization works", {
diff --git a/R-package/tests/testthat/test_callbacks.R b/R-package/tests/testthat/test_callbacks.R
index 9016c1bcbdfa..fd42f519af60 100644
--- a/R-package/tests/testthat/test_callbacks.R
+++ b/R-package/tests/testthat/test_callbacks.R
@@ -173,16 +173,16 @@ test_that("cb.reset.parameters works as expected", {
 })
 
 test_that("cb.save.model works as expected", {
-  files <- c('xgboost_01.model', 'xgboost_02.model', 'xgboost.model')
+  files <- c('xgboost_01.json', 'xgboost_02.json', 'xgboost.json')
   for (f in files) if (file.exists(f)) file.remove(f)
 
   bst <- xgb.train(param, dtrain, nrounds = 2, watchlist, eta = 1, verbose = 0,
-                   save_period = 1, save_name = "xgboost_%02d.model")
-  expect_true(file.exists('xgboost_01.model'))
-  expect_true(file.exists('xgboost_02.model'))
-  b1 <- xgb.load('xgboost_01.model')
+                   save_period = 1, save_name = "xgboost_%02d.json")
+  expect_true(file.exists('xgboost_01.json'))
+  expect_true(file.exists('xgboost_02.json'))
+  b1 <- xgb.load('xgboost_01.json')
   expect_equal(xgb.ntree(b1), 1)
-  b2 <- xgb.load('xgboost_02.model')
+  b2 <- xgb.load('xgboost_02.json')
   expect_equal(xgb.ntree(b2), 2)
 
   xgb.config(b2) <- xgb.config(bst)
@@ -191,9 +191,9 @@ test_that("cb.save.model works as expected", {
 
   # save_period = 0 saves the last iteration's model
   bst <- xgb.train(param, dtrain, nrounds = 2, watchlist, eta = 1, verbose = 0,
-                   save_period = 0)
-  expect_true(file.exists('xgboost.model'))
-  b2 <- xgb.load('xgboost.model')
+                   save_period = 0, save_name = 'xgboost.json')
+  expect_true(file.exists('xgboost.json'))
+  b2 <- xgb.load('xgboost.json')
   xgb.config(b2) <- xgb.config(bst)
   expect_equal(bst$raw, b2$raw)