mWebView.setOnKeyListener(new OnKeyListener() {
    public boolean onKey(View v,int keyCode,KeyEvent event) {
        switch(keyCode)
        {
            case KeyEvent.KEYCODE_BACK:
                if(mWebView.canGoBack() == true){
                    mWebView.goBack();
                    return true;
                } else
                    return false;   
                }

                return false;
            }
        });