Type 2 ryanodine receptor (RyR2) is a Ca2+ release channel on the sarcoplasmic reticulum and plays a pivotal role in excitationcontraction coupling in heart. RyR2 is the major target for arrhythmogenic diseases such as catecholaminergic polymorphic ventricular tachycardia (CPVT), idiopathic ventricular fibrillation (IVF), long QT syndrome (LQTS), etc. We have characterized various types of arrhythmogenic RyR2 mutations using HEK cell expression system and found that some of IVF and LQTS mutants reveal loss-of-function phenotype whereas others are gain-of-function mutants. In this study, we aimed to compare the effects of loss-of-function and gain-of-function RyR2 mutants on Ca2+ signaling in cardiac intracellular milieu of HL-1 cells, mouse cardiac cell line. Wild type (WT), 4 loss-of-function type and 2 gain-of-function type mutant RyR2s were expressed in HL-1 cells using baculovirus system and intracellular Ca2+ signals were monitored with Cal520. Under semiconfluent condition, most of HL-1 cells did not show spontaneous beating. Expression of WT and gain-of-function mutants in such cells showed frequent Ca2+ waves whereas loss-of-function mutants exhibited no Ca2+ waves. On the other hand, cells forming a cluster under confluent condition showed spontaneous, highly rhythmic, and synchronized Ca2+ transients caused by automatic activity. Expression of gain-of-function mutants in the beating cells caused frequent Ca2+ waves during spontaneous beating, while loss-of-function mutants reduced amplitudes of Ca2+ transients. Mechanism of perturbation of Ca2+ signals in HL-1 cells will be discussed based on the distribution of endogenous and exogenous RyR2s.