The effects of Co on the transition and the deformation characteristics of Ti-49.8Ni shape memory alloy were investigated by thermal gravity, X-ray diffraction, differential scanning calorimetry and tensile test. The results indicate that the phase transformation types of intermediate-temperature annealed Ti-49.8Ni alloy during cooling/heating cycle is A→R→M/M→A (A—parent phase, R—R phase, M—martensite phase). With the annealing temperature increasing, the M-phase transformation temperature of Ti-49.8Ni alloy increases, and the R-phase transition temperature increases first and then decreases. The phase of Ti-49.8Ni alloy at room temperature is martensite and shows shape memory effect (SME). After adding 1% Co, the transformation type of Ti-49.8Ni-1Co alloy is A→R→M/M→R→A upon cooling/heating, and the transition temperature of the alloy is low. The phase of Ti-49.8Ni-1Co alloy at room temperature is parent phase, and shows superelasticity (SE). The excellent SME and SE can be obtained in Ti-Ni base alloys when the annealing temperature is below 600 ℃. When the annealing temperature is over 600℃, the oxidation of Ti-Ni alloys speeds up, SME and SE become bad, but the plasticity of the alloy is improved significantly