Assessment of resource holding potential (RHP) of their own and that of oppornents affects the animal behaviours in contest competition. Contestants may use their own RHP and/or the relative RHP of opponents to make decision and may switch these two assessment tactic during fighting. This study examined the indices and tactics of assessment of RHP in male-male contest of hermit crabs Pagurus middendorffii . Males of this species show precopulatory guarding behaviour, and male-male contest often occur when the males guarding females encounter with intruder male. I conducted trials of dyadic contest to investigate the assessment tactics in two phases of the contest; before and during physical combat. Intruders gave up the contest without escalating to physical combats when own body size was smaller irrespective of that of guarding males. When a physical combat occurred, the duration of combat was longer and the guarding males had higher possibility of loose the females if the difference of the size of major cheliped between guarding males and intruders was smaller. These suggest that P. middendorffii possibly use own body size before physical combat and relative major cheliped size during combat to assess RHP.