Elliptic curve cryptography (ECC) has been widely used for the digital signature to ensure the secu-rity in communication. It is important for the ECC processor to support a variety of ECC standards to be compatible with different security applications. Thus, a flexible proces-sor which can support different standards and algorithms is desired. In this paper, an efficient and flexible dual-field ECC processor using the hardware-software approach is presented. The proposed processor can support arbitrary elliptic curve. An elaborate modular arithmetic logic unit is designed. It can perform basic modular arithmetic opera-tions and achieve high efficiency. Based on our designed instruction set, the processor can be programmed to perform various point operations based on different algorithms. To demonstrate the flexibility of our processor, a pointmultipli-cation algorithm with power analysis resistance is adopted. Our design is implemented in the field-programmable gate array platform and also in the application-specified inte-grated circuit. After implemented in the 55 nm CMOS process, the processor takes between 0.60 ms (163 bits ECC) and 6.75ms (571 bits ECC) to finish one-pointmultiplication. Compared to other related works, the merits of our ECC pro-cessor are the high hardware efficiency and flexibility.