The earlier experimental results showed that the luminescent characteristics of Tb

3+

ion depend on Tb

3+

ion concentration and crystal hosts.In this paper theoretical investigation of Tb

3+

luminescent characteritics in pentaphosphate and mixing pentaphosphate single crystals (TbP

5

O

14

and Tb

x

Y

1-x

P

5

O

14

) has been carried out. By using the Judd-Ofelt theory and absorption spectra

the parameters of the oscillator strengh were first obtained in TbP

5

O

15

crystals

resulting in O

2

=2.13×10

-20

cm

2

O

4

=3.76×10

-20

cm

2

O

6

=2.98×10

-20

cm

2

.The probabilities of electric dipole and magnetic dipole radiative transition from

5

D

3

and

5

D

4

to

7

F

J

were calculated. The lifetime of electron in

5

D

4

obtained is 4.4ms

which agrees with experimental value.