diff --git a/Network/ByteOrder.hs b/Network/ByteOrder.hs
--- a/Network/ByteOrder.hs
+++ b/Network/ByteOrder.hs
@@ -45,6 +45,7 @@
   , read16
   , read24
   , read32
+  , read64
   , extractByteString
   , extractShortByteString
     -- *Writing to buffer
@@ -56,6 +57,7 @@
   , write16
   , write24
   , write32
+  , write64
   , copyByteString
   , copyShortByteString
   , shiftLastN
@@ -375,11 +377,11 @@
 write8 :: WriteBuffer -> Word8 -> IO ()
 write8 WriteBuffer{..} w = do
     ptr <- readIORef offset
-    if ptr >= limit then
+    let ptr' = ptr `plusPtr` 1
+    if ptr' > limit then
         throwIO BufferOverrun
       else do
         poke ptr w
-        let ptr' = ptr `plusPtr` 1
         writeIORef offset ptr'
 
 {-# INLINE write16 #-}
@@ -391,11 +393,11 @@
 write16 :: WriteBuffer -> Word16 -> IO ()
 write16 WriteBuffer{..} w = do
     ptr <- readIORef offset
-    if ptr `plusPtr` 1 >= limit then
+    let ptr' = ptr `plusPtr` 2
+    if ptr' > limit then
         throwIO BufferOverrun
       else do
         poke16 w ptr 0
-        let ptr' = ptr `plusPtr` 2
         writeIORef offset ptr'
 
 {-# INLINE write24 #-}
@@ -407,11 +409,11 @@
 write24 :: WriteBuffer -> Word32 -> IO ()
 write24 WriteBuffer{..} w = do
     ptr <- readIORef offset
-    if ptr `plusPtr` 2 >= limit then
+    let ptr' = ptr `plusPtr` 3
+    if ptr' > limit then
         throwIO BufferOverrun
       else do
         poke24 w ptr 0
-        let ptr' = ptr `plusPtr` 3
         writeIORef offset ptr'
 
 {-# INLINE write32 #-}
@@ -423,13 +425,26 @@
 write32 :: WriteBuffer -> Word32 -> IO ()
 write32 WriteBuffer{..} w = do
     ptr <- readIORef offset
-    if ptr `plusPtr` 3 >= limit then
+    let ptr' = ptr `plusPtr` 4
+    if ptr' > limit then
         throwIO BufferOverrun
       else do
         poke32 w ptr 0
-        let ptr' = ptr `plusPtr` 4
         writeIORef offset ptr'
 
+{-# INLINE write64 #-}
+-- | Write four bytes and ff one byte.
+--   If buffer overrun occurs, 'BufferOverrun' is thrown.
+write64 :: WriteBuffer -> Word64 -> IO ()
+write64 WriteBuffer{..} w = do
+    ptr <- readIORef offset
+    let ptr' = ptr `plusPtr` 8
+    if ptr' > limit then
+        throwIO BufferOverrun
+      else do
+        poke64 w ptr 0
+        writeIORef offset ptr'
+
 {-# INLINE shiftLastN #-}
 -- | Shifting the N-bytes area just before the current pointer (the 3rd argument).
 --   If the second argument is positive, shift it to right.
@@ -701,6 +716,13 @@
     w32 <- withCurrentOffSet rbuf (`peek32` 0)
     ff rbuf 4
     return w32
+
+-- | Reading four bytes as 'Word64' and ff four bytes.
+read64 :: Readable a => a -> IO Word64
+read64 rbuf = do
+    w64 <- withCurrentOffSet rbuf (`peek64` 0)
+    ff rbuf 8
+    return w64
 
 -- | Buffer overrun exception.
 data BufferOverrun = BufferOverrun -- ^ The buffer size is not enough
diff --git a/network-byte-order.cabal b/network-byte-order.cabal
--- a/network-byte-order.cabal
+++ b/network-byte-order.cabal
@@ -2,7 +2,7 @@
 -- documentation, see http://haskell.org/cabal/users-guide/
 
 name:                network-byte-order
-version:             0.1.2.1
+version:             0.1.3.0
 synopsis:            Network byte order utilities
 description:         Peek and poke functions for network byte order.
 license:             BSD3