HAP@AEPH 2 -SO 3 H as a new, green, solid acid catalyst was prepared, characterized and applied for the synthesis of 4,4'-( aryl methylene)bis(3-methyl-1H-pyrazol-5-ol)s. Abstract: Sulfonated nanohydroxyapatite functionalized with 2-aminoethyl dihydrogen phosphate (HAP@AEPH 2 -SO 3 H) as a new green, recyclable solid acid catalyst was prepared and characterized by FT-IR, XRD, SEM, TEM and TGA/DTA techniques. Composition of HAP@AEPH 2 -SO 3 H was determined as nonohydroxyapatite, while particles were observed to have nanorods morphology.Size estimations using TEM (10-100 nm) and the crystallinity using XRD (hexagonal phase) are well-matched to character of nanohydroxyapatite. The catalytic activity of HAP@AEPH 2 -SO 3 H was evaluated for synthesis of 4,4'-(aryl methylene)bis(3methyl-1H-pyrazol-5-ol)s via one-pot reactions of phenylhydrazine/or hydrazine hydrate, ethylacetoacetate and aldehydes under solvent-free condition. This catalyst showed notable advantages such as environmental friendliness, excellent yields, shorter reaction time, reusability of inexpensive catalyst and easy workup procedure.