In metal directed energy deposition (DED), defects such as porosity, lack-of-fusion and cracking often occur during its material melting-solidification process. In this research, a femtosecond laser based transient thermoreflectance (TTR) technique was developed for porosity inspection in metal DED. The major contributions of the proposed technique include the following: (1) A femtosecond laser based TTR measurement system is developed for fully noncontact measurement of thermoreflectance from a newly deposited layer in DED. (2) Porosity is inspected by comparing the thermoreflectance measured at different thermal diffusion length. (3) Due to the noncontact nature and scanning capability, the proposed porosity inspection technique can be readily applied to online porosity monitoring during the DED process. It is inferred that, together with instantaneous correction actions, enhanced quality of the manufactured objects can be achieved. In this paper, offline validation tests were performed on Ti-6Al-4V samples manufactured with different DED printing parameters.