diff --git a/src/Utils.php b/src/Utils.php
index fbcf188..c0be691 100644
--- a/src/Utils.php
+++ b/src/Utils.php
@@ -282,6 +282,25 @@ class Utils
         }
         $bnt = new BigNumber(self::UNITS[$unit]);
 
+        if (is_array($bn)) {
+            // fraction number
+            list($whole, $fraction, $negative1) = $bn;
+
+            $fractionLength = strlen($fraction->toString());
+
+            if ($fractionLength > strlen(self::UNITS[$unit])) {
+                throw new InvalidArgumentException('toWei fraction part is out of limit.');
+            }
+            $whole = $whole->multiply($bnt);
+            $base = (new BigNumber(10))->pow(new BigNumber($fractionLength));
+            $fraction = $fraction->multiply($bnt)->divide($base)[0];
+
+            if ($negative1 !== false) {
+                return $whole->add($fraction)->multiply($negative1);
+            }
+            return $whole->add($fraction);
+        }
+
         return $bn->multiply($bnt);
     }
 
diff --git a/test/unit/UtilsTest.php b/test/unit/UtilsTest.php
index a519ab4..50b3f0e 100644
--- a/test/unit/UtilsTest.php
+++ b/test/unit/UtilsTest.php
@@ -259,14 +259,51 @@ class UtilsTest extends TestCase
         $bn = Utils::toWei('0x5218', 'wei');
         $this->assertEquals($bn->toString(), '21016');
 
+        $bn = Utils::toWei('0.1', 'ether');
+        $this->assertEquals($bn->toString(), '100000000000000000');
+
+        $bn = Utils::toWei('1.69', 'ether');
+        $this->assertEquals($bn->toString(), '1690000000000000000');
+
+        $bn = Utils::toWei(0.1, 'ether');
+        $this->assertEquals($bn->toString(), '100000000000000000');
+
+        $bn = Utils::toWei(1.69, 'ether');
+        $this->assertEquals($bn->toString(), '1690000000000000000');
+
+        $bn = Utils::toWei('-0.1', 'ether');
+        $this->assertEquals($bn->toString(), '-100000000000000000');
+
+        $bn = Utils::toWei('-1.69', 'ether');
+        $this->assertEquals($bn->toString(), '-1690000000000000000');
+
+        $bn = Utils::toWei(-0.1, 'ether');
+        $this->assertEquals($bn->toString(), '-100000000000000000');
+
+        $bn = Utils::toWei(-1.69, 'ether');
+        $this->assertEquals($bn->toString(), '-1690000000000000000');
+
+        $bn = Utils::toWei('', 'ether');
+        $this->assertEquals($bn->toString(), '0');
+
+        $bn = Utils::toWei(-1.697, 'kwei');
+        $this->assertEquals($bn->toString(), '-1697');
+
         try {
-            $toWei = Utils::toWei('0x5218', new stdClass);
+            $bn = Utils::toWei('0x5218', new stdClass);
         } catch (InvalidArgumentException $e) {
             $this->assertTrue($e !== null);
         }
 
         try {
-            $toWei = Utils::toWei('0x5218', 'test');
+            $bn = Utils::toWei('0x5218', 'test');
+        } catch (InvalidArgumentException $e) {
+            $this->assertTrue($e !== null);
+        }
+
+        try {
+            // out of limit
+            $bn = Utils::toWei(-1.6977, 'kwei');
         } catch (InvalidArgumentException $e) {
             $this->assertTrue($e !== null);
         }