Background: H-reflex is a valuable electrophysiological technique for assessing nerve conduction through entire length of afferent and efferent pathways, especially nerve roots and proximal segments of peripheral nerves. The aim of this study was to investigate the relation between normal values of flexor carpi radialis (FCR) H-reflex latency, upper limb length and age in normal subjects, and to determine whether there is any regression equation between them. Methods: By considering the criteria of inclusion and exclusion, 120 upper limbs of 69 normal volunteers (68 hands of 39 men and 52 hands of 30 women) with the mean age of 39. 8 ± 11. 2 years participated in this study. FCR H-reflex was obtained by standard electrodiagnostic techniques, and its onset latency was recorded. Upper limb length and arm length were measured in defined position. The degree of association between these variables was determined with Pearson correlation and linear regression was used for obtaining the proposed relations. Results: Mean FCR H-reflex latency was found to be 15. 88 ± 1. 27 ms. There was a direct linear correlation between FCR H-reflex latency and upper limb length (r = 0. 647) and also arm length (r = 0. 574), but there was no significant correlation between age and FCR H-reflex latency (P = 0. 260). Finally, based on our findings, we tried to formulate these relations by statistical methods. Conclusion: We found that upper limb length and arm length are good predictive values for estimation of normal FCR H-reflex latency but age, in the range of 20-60 years old, has no correlation with its latency. This estimation could have practical indications in pathologic conditions.