10. Battle of Tannenberg (182,000 total casualties)
9. Battle of Arras (278,000 total casualties)
8. Battle of Gallipoli (473,000 total casualties)
7. First Battle of the Marnes (483,000 total casualties)
6. Serbian Campaign (633,500+ total casualties)
5. Battle of Passchendaele (848,614 total casualties estimated)
4. Battle of Verdun (976,000 total casualties)
3. Battle of the Somme (1,219,201 total casualties)
2. Spring Offensive (1,539,715 total casualties)
1. Hundred Days Offensive (1,855,369 total casualties)
source: http://www.toptenz.net/top-10-bloodiest-battles-of-world-war-i.php