diff --git a/src/core/index.rs b/src/core/index.rs
index ed254c393e..a683fd6aaa 100644
--- a/src/core/index.rs
+++ b/src/core/index.rs
@@ -13,7 +13,7 @@ use crate::directory::INDEX_WRITER_LOCK;
 use crate::directory::{Directory, RamDirectory};
 use crate::error::DataCorruption;
 use crate::error::TantivyError;
-use crate::indexer::index_writer::HEAP_SIZE_MIN;
+use crate::indexer::index_writer::{HEAP_SIZE_MIN, MAX_NUM_THREAD};
 use crate::indexer::segment_updater::save_new_metas;
 use crate::reader::IndexReader;
 use crate::reader::IndexReaderBuilder;
@@ -432,7 +432,8 @@ impl Index {
 
     /// Creates a multithreaded writer
     ///
-    /// Tantivy will automatically define the number of threads to use.
+    /// Tantivy will automatically define the number of threads to use, but
+    /// no more than [`MAX_NUM_THREAD`] threads.
     /// `overall_heap_size_in_bytes` is the total target memory usage that will be split
     /// between a given number of threads.
     ///
@@ -441,7 +442,7 @@ impl Index {
     /// # Panics
     /// If the heap size per thread is too small, panics.
     pub fn writer(&self, overall_heap_size_in_bytes: usize) -> crate::Result<IndexWriter> {
-        let mut num_threads = num_cpus::get();
+        let mut num_threads = std::cmp::min(num_cpus::get(), MAX_NUM_THREAD);
         let heap_size_in_bytes_per_thread = overall_heap_size_in_bytes / num_threads;
         if heap_size_in_bytes_per_thread < HEAP_SIZE_MIN {
             num_threads = (overall_heap_size_in_bytes / HEAP_SIZE_MIN).max(1);
diff --git a/src/indexer/index_writer.rs b/src/indexer/index_writer.rs
index 11b273016d..eac379ca4c 100644
--- a/src/indexer/index_writer.rs
+++ b/src/indexer/index_writer.rs
@@ -43,6 +43,9 @@ pub const MARGIN_IN_BYTES: usize = 1_000_000;
 pub const HEAP_SIZE_MIN: usize = ((MARGIN_IN_BYTES as u32) * 3u32) as usize;
 pub const HEAP_SIZE_MAX: usize = u32::max_value() as usize - MARGIN_IN_BYTES;
 
+// We impose the number of index writter thread to be at most this.
+pub const MAX_NUM_THREAD: usize = 8;
+
 // Add document will block if the number of docs waiting in the queue to be indexed
 // reaches `PIPELINE_MAX_SIZE_IN_DOCS`
 const PIPELINE_MAX_SIZE_IN_DOCS: usize = 10_000;