Y

2

O

3

:Ho/Yb nanocrystals have been prepared by combustion method. The nano-crystals size was estimated by X-ray diffraction. Green emissions near 549 nm and red emissions near 668 nm were observed under 488 nm and 980 nm excitation

which were associated with

5

S

2

(

5

F

4

)

5

I

8

and

5

F

5

5

I

8

transitions of Ho

3+

that required two-step energy transfer (ET) from Yb

3+

ions. The intensity ratio of red light to green light varied obviously with the nanocrystals size

which should be due to surface defects. The smaller the size was

the weaker the emission intensity became. As the same time

the ratio of red light intensity to green light intensity increased obviously with the decrease of particle size. As the particle size decreased

a great number of surface defects were involved. These defects

which acted as nonradiative transition channels

led the decrease of the emission intensity. On the other hand

the surface of the powders absorbed the CO

2

and the H

2

Oin the air. So large vibrational modes could be produced

which offered the probability for depopulating of the excited states nonradiatively. Thus

the nonradiative relaxation rates from

5

S

2

(

5

F

4

) to

5

F

5

became much faster. As a result the ratio of red light intensity to green light intensity decreased. Two nonradiative relaxations processes

5

S

2

(

5

F

4

)

5

F

5

and

5

I

6

5

I

7

are involved in upconversion luminescence

but only

5

S

2

(

5

F

4

)

5

F

5

in downconversion. So the increase of emission intensity ratio is more prominent in upconversion luminescence than downconversion luminescence.