The objective of this study was to assess the accuracy of two frequency apex locators, Root ZX (Morita, Kyoto, Japan) and Endex (Osada, Tokyo, Japan) in primary teeth with and without root resorption in vitro. For this study, 90 sound extracted primary teeth (60 molars and 30 incisors; 93 roots with visible resorption and 51 roots without) were selected. A total of 144 root canals were included. After access preparation, root canal lengths were measured visually. The teeth were embedded in an alginate model developed specifically for testing apex locators. Electronic length was determined with two different frequency apex locators, Root ZX (Morita, Kyoto, Japan) and Endex (Osada, Tokyo, Japan). Statistical evaluation was performed using Mann-Whitney U and Wilcoxon W tests (p < 0.05). Results revealed that both apex locator devices did not show similar values to visual length measurements with statistically significant differences in primary teeth with root resorption (p < 0.05). For the teeth without root resorption, Endex showed similar values to visual length measurements with no statistically significant differences (p > 0.05), but Root ZX values were different than visual length and the differences were statistically significant (p < 0.05). There were no significant differences between the two apex locators considering the presence of root resorption. The two frequency apex locators can be recommended for use in root canal therapy for primary teeth with and without root resorption, only if electrical determination of root canal length is supported with other diagnostic measures.