diff --git a/include/fluent-bit/flb_simd.h b/include/fluent-bit/flb_simd.h
index 19e212c67b6..de789624ba6 100644
--- a/include/fluent-bit/flb_simd.h
+++ b/include/fluent-bit/flb_simd.h
@@ -100,6 +100,15 @@ typedef uint64_t flb_vector8;
 typedef uint8_t flb_vector8;
 #endif /* FLB_SIMD_DISABLED */
 
+/* RVV's instruction length is flexible and not fixed width.
+ * We assumed that VLEN which is the fundamental intsruction length is 128.
+ */
+#if defined(FLB_SIMD_RVV)
+#define FLB_SIMD_VEC8_INST_LEN RVV_VEC8_INST_LEN
+#else
+#define FLB_SIMD_VEC8_INST_LEN sizeof(flb_vector8)
+#endif
+
 /* element-wise comparisons to a scalar */
 static inline bool flb_vector8_has(const flb_vector8 v, const uint8_t c);
 static inline bool flb_vector8_has_zero(const flb_vector8 v);
diff --git a/src/flb_utils.c b/src/flb_utils.c
index 20d09b04d5c..21cee6268d6 100644
--- a/src/flb_utils.c
+++ b/src/flb_utils.c
@@ -801,11 +801,7 @@ int flb_utils_write_str(char *buf, int *off, size_t size, const char *str, size_
     off_t offset = 0;
     char tmp[16];
     char *p;
-#if defined(FLB_SIMD_RVV)
-    const size_t inst_len = RVV_VEC8_INST_LEN;
-#else
-    const size_t inst_len = sizeof(flb_vector8);
-#endif
+    const size_t inst_len = FLB_SIMD_VEC8_INST_LEN;
 
     /* to encode codepoints > 0xFFFF */
     uint16_t high;