The aim of this study was to measure the cusp deflection during composite restoration for MOD cavity in premolar and to examine the influence of cavity dimension, C-factor and restoration method on the cusp deflection. Thirty extracted maxillary premolar were prepared to four different sizes of MOD cavity and divided into six groups. The width and depth of the cavity were as follows. Group 1; $1.5{;}{ imes}{;}1{;}mm$, Group 2; $1.5{;}{ imes}{;}2{;}mm$, Group 3; $3{;}{ imes}{;}1{;}mm$, and Group 4-6; $3{;}{ imes}{;}2{;}mm$ respectively. Group 1-4 were restored using bulk filling method with Z-250 composite. However, Group 5 was restored incrementally, and Group 6 was restored with an indirect resin inlay. The cusp deflection was recorded at the buccal and lingual cusp tips using LVDT probe for 10,000 seconds. The measured cusp deflections were compared between groups, and the relationship between the cube of the length of cavity wall/the cube of the thickness of cavity wall ($L^3/T^3$). C-factor and cusp deflection or % flexure ($100{;}{ imes}$ cuspal deflection / cavity width) was analyzed. The cusp deflection of Group 1-4 were $12.1{;}mu extrm{m},{;}17.2{;}mu extrm{m},{;}16.2{;}mu extrm{m}{;}and{;}26.4{;}mu extrm{m}$ respectively. The C-factor was related to the % flexure rather than the cusp deflection. There was a strong positive correlationship between the $L^3/T^3$ and the cusp deflection. The cusp deflection of Group 5 and 6 were $17.4{;}mu extrm{m}{;}and{;}17.9{;}mu extrm{m}$ respectively, which are much lower value than that of Group 4.