Conditions were investigated for preparing needle-like hydroxyapatite (HAp; Ca₁₀(PO₄)₆(OH)₂) by hydrothermal processing of mixed calcium phosphate powders consisting of dicalcium phosphate dihydrate (DCPD; CaHPO₄·2H₂O) and beta-tricalcium phosphate (β-TCP; β-Ca₃(PO₄)₂). The powder mixture was subjected to two-step hydrothermal processing in which the compacted powder was treated first by exposure to vapor of the liquid and subsequently by immersion in the liquid. High aspect ratios were obtained in samples with crystal sizes around 60 μm in length from the reactant mass ratio composition of β-TCP/DCPD = 1/9 by treatment at 160°C for 0.5 h under vapor conditions, followed by liquid conditions for 24 h with renewal of the liquid at 12 h. Initial compositions and pH conditions were the main parameters determining the formation of needle-like HAp by hydrothermal processing.