Nanomedicine can kill tumor cells or block the growth and spread of cancers by maximizing drug accumulation in tumor tissues. How much drug will selectively accumulate in tumor tissues depends on physical parameters such as vessel permeability in tumor microenvironments. On the other hand, by properly choosing the size of nanomedicine, the accumulation of drugs in normal tissues will be reduced. In this study, we measured the permeability rate of vessels in different strain of mice in vivo with two-photon fluorescent angiography. We used FITC / TRITC / TEXAS RED labeled dextran to investigate size-dependent permeation from vessels in normal tissues. We choose nanoparticles of 40kDa, 70 kDa, 150 kDa and 2000 kDa, and selected three strains of wild-type Balb/C, ICR, C2J mice. We found , even at the same size, the vascular permeability rate of dextran still vary with the dye-conjugates and the strain of mice. One of our results shows that ICR mice have the biggest vessel hole in normal tissues. Choosing wrong nanoparticle size will cause their leaking in normal tissues. This study will provide a new threshold and guideline to reduce the accumulation of nanomedicine in normal tissues.