This study investigated the effects of working memory load (WML) and automaticity on mental addition through an examination of both task and individual characteristics within the framework of cognitive load theory. Results from 73 fourth graders showed that WML, automaticity, and their interaction had significant effects on mental addition. Automaticity had a differential effect on response time under low and high WML conditions. Results also showed that working memory, math fluency, and their interaction could predict a significant portion of variance in accuracy. However, only math fluency could predict mental addition speed. The study confirmed the interaction effect between working memory and automaticity and underscored the importance of automaticity in arithmetic learning.