diff --git a/modules/math/math.c b/modules/math/math.c
index bd09962..decae18 100644
--- a/modules/math/math.c
+++ b/modules/math/math.c
@@ -16,6 +16,14 @@ static void PH7_M_PI_Const(ph7_value *pVal, void *pUserData) {
 	SXUNUSED(pUserData); /* cc warning */
 	ph7_value_double(pVal, PH7_PI);
 }
+/*
+ * M_TAU
+ *  Expand the value of tau.
+ */
+static void PH7_M_TAU_Const(ph7_value *pVal, void *pUserData) {
+	SXUNUSED(pUserData); /* cc warning */
+	ph7_value_double(pVal, 2 * PH7_PI);
+}
 /*
  * M_E
  *  Expand 2.7182818284590452354
diff --git a/modules/math/math.h b/modules/math/math.h
index d97a857..60d2e81 100644
--- a/modules/math/math.h
+++ b/modules/math/math.h
@@ -22,6 +22,7 @@
 
 /* Forward reference & declaration */
 static void PH7_M_PI_Const(ph7_value *pVal, void *pUserData);
+static void PH7_M_TAU_Const(ph7_value *pVal, void *pUserData);
 static void PH7_M_E_Const(ph7_value *pVal, void *pUserData);
 static void PH7_M_LOG2E_Const(ph7_value *pVal, void *pUserData);
 static void PH7_M_LOG10E_Const(ph7_value *pVal, void *pUserData);
@@ -65,6 +66,7 @@ PH7_PRIVATE sxi32 initializeModule(ph7_vm *pVm, ph7_real *ver, SyString *desc);
 
 static const ph7_builtin_constant mathConstList[] = {
 	{"M_PI",                 PH7_M_PI_Const         },
+	{"M_TAU",                PH7_M_TAU_Const        },
 	{"M_E",                  PH7_M_E_Const          },
 	{"M_LOG2E",              PH7_M_LOG2E_Const      },
 	{"M_LOG10E",             PH7_M_LOG10E_Const     },