The relationship between primary dendrite arm spacing and secondary one was investigated in experiment, and was found that the latter is roughly one half of the former. Theoretically, the primary arm spacing is interpreted by thermodynamic perturbations ahead of the primary tip. On the contrary growth mechanism of secondary arm is ascribed to Ostwald-ripening and/or sintering/coalescence mechanism. The experimental result is analysed by the mechanisms, and is shown the 'one half relationship' is a good approximation.