In　watermarking　schemes,　image　is　considered　as　a　communication　channel　to　transmit　messages.　But　because　of　the　requirements　of　watermarking　robustness　and　visibility,　Watermarking　have　some　characteristics　different　from　traditional　communication.　Watermarking　capacity　should　be　associated　with　the　content　of　image;　different　image　has　different　watermarking　capacity.　In　this　paper,　we　present　a　content　adaptive　watermarking　algorithm　to　embed　the　strongest　watermark　while　keeping　the　watermark＇s　invisibility,　and　discuss　watermarking　capacity　in　this　scenario.　We　also　derive　the　relation　between　watermarking　capacity　and　detection　bit　error　rate　(BER)　for　the　first　time.