Elliptic curve cryptography processor proposed as the next generation public key cryptosystem was designed. The scalable ECC processor supports multiple elliptic curves of P-192, P-224, P-256, and P-384 over prime field as defined by the NIST FIPS 186-2 standard. The processor consist of Smul_Mem block to store intermediate results of scalar multiplication, SAlu_GFp block to perform addition, subtraction, multiplication, and squaring over prime field of various lengths, and a control block. The modified Montgomery ladder algorithm was used to perform scalar multiplication. In order to eliminate division operations that require large hardware, the projective coordinate was adopted to implement point operations. The final result of the scalar multiplication operation is translated to affine coordinate system for compatibility with other processors. The operations such as addition, subtraction, multiplication are carried out over prime field. It was designed with a 32-bit word-based structure to complete all operations on fixed hardware resources irrespective of field size. A new arithmetic circuit that is based on 32-bit data path was devised to implement Montgomery multiplier over prime field. The designed Montgomery multiplier effectively reduced hardware complexity and critical path by using the regularity of the algorithm. The comparator required for Montgomery multiplier is replaced with designed adder/subtractor to reduce hardware resource. The scalable ECC processor was verified by functional verification using Modelsim and FPGA implementation. The processor synthesized using a 0.18 CMOS cell library occupies 5.4-Kbit RAM and 26,283 gate equivalents (GEs), and the estimated maximum clock frequency is 125 MHz. A scalar multiplication takes 0.6, 0.9, 1.2, 3.7 million clock cycles for P-192, P-224, P-256, P-384, respectively. It is expected to support scalar multiplication on multiple elliptic curves using a single processor and to be applied to an information security system requiring various security strengths.