diff --git a/lib/SSIMCriterion.lua b/lib/SSIMCriterion.lua
new file mode 100644
index 0000000..9137920
--- /dev/null
+++ b/lib/SSIMCriterion.lua
@@ -0,0 +1,67 @@
+-- SSIM Index, ref: http://www.cns.nyu.edu/~lcv/ssim/ssim_index.m
+local SSIMCriterion, parent = torch.class('w2nn.SSIMCriterion','nn.Criterion')
+function SSIMCriterion:__init(ch, kernel_size, sigma)
+   parent.__init(self)
+   local function gaussian2d(kernel_size, sigma)
+      sigma = sigma or 1
+      local kernel = torch.Tensor(kernel_size, kernel_size)
+      local u = math.floor(kernel_size / 2) + 1
+      local amp = (1 / math.sqrt(2 * math.pi * sigma^2))
+      for x = 1, kernel_size do
+	 for y = 1, kernel_size do
+	 kernel[x][y] = amp * math.exp(-((x - u)^2 + (y - u)^2) / (2 * sigma^2))
+	 end
+      end
+      kernel:div(kernel:sum())
+      return kernel
+   end
+   ch = ch or 1
+   kernel_size = kernel_size or 11
+   sigma = sigma or 1.5
+   local kernel = gaussian2d(kernel_size, sigma)
+   if ch > 1 then
+      local kernel_nd = torch.Tensor(ch, ch, kernel_size, kernel_size)
+      for i = 1, ch do
+	 for j = 1, ch do
+	    kernel_nd[i][j]:copy(kernel)
+	    if i ~= j then
+	       kernel_nd[i][j]:zero()
+	    end
+	 end
+      end
+      kernel = kernel_nd
+   end
+   self.c1 = 0.01^2
+   self.c2 = 0.03^2
+   self.ch = ch
+   self.conv = nn.SpatialConvolution(ch, ch, kernel_size, kernel_size, 1, 1, 0, 0):noBias()
+   self.conv.weight:copy(kernel)
+   self.mu1 = torch.Tensor()
+   self.mu2 = torch.Tensor()
+   self.mu1_sq = torch.Tensor()
+   self.mu2_sq = torch.Tensor()
+   self.mu1_mu2 = torch.Tensor()
+   self.sigma1_sq = torch.Tensor()
+   self.sigma2_sq = torch.Tensor()
+   self.sigma12 = torch.Tensor()
+   self.ssim_map = torch.Tensor()
+end
+function SSIMCriterion:updateOutput(input, target)-- dynamic range: 0-1
+   assert(input:nElement() == target:nElement())
+   local valid = self.conv:forward(input)
+   self.mu1:resizeAs(valid):copy(valid)
+   self.mu2:resizeAs(valid):copy(self.conv:forward(target))
+   self.mu1_sq:resizeAs(self.mu1):copy(self.mu1):cmul(self.mu1)
+   self.mu2_sq:resizeAs(self.mu2):copy(self.mu2):cmul(self.mu2)
+   self.mu1_mu2:resizeAs(self.mu1):copy(self.mu1):cmul(self.mu2)
+   self.sigma1_sq:resizeAs(valid):copy(self.conv:forward(torch.cmul(input, input)):add(-1, self.mu1_sq))
+   self.sigma2_sq:resizeAs(valid):copy(self.conv:forward(torch.cmul(target, target)):add(-1, self.mu2_sq))
+   self.sigma12:resizeAs(valid):copy(self.conv:forward(torch.cmul(input, target)):add(-1, self.mu1_mu2))
+
+   local ssim = self.mu1_mu2:mul(2):add(self.c1):cmul(self.sigma12:mul(2):add(self.c2)):
+      cdiv(self.mu1_sq:add(self.mu2_sq):add(self.c1):cmul(self.sigma1_sq:add(self.sigma2_sq):add(self.c2))):mean()
+   return ssim
+end
+function SSIMCriterion:updateGradInput(input, target)
+   error("not implemented")
+end
diff --git a/lib/w2nn.lua b/lib/w2nn.lua
index 8d28c69..6d047d7 100644
--- a/lib/w2nn.lua
+++ b/lib/w2nn.lua
@@ -30,6 +30,7 @@ else
    require 'LeakyReLU'
    require 'ClippedWeightedHuberCriterion'
    require 'ClippedMSECriterion'
+   require 'SSIMCriterion'
    require 'InplaceClip01'
    return w2nn
 end