A new kind of sandwich-like Praseodymium bis complex Pr(Pc

*

)

2

(Pc

*

=Pc(OC

8

H

17

)

8

;R=OC

8

H

17

)is used as film-forming material.Mixed ultrathin films of 1:3 Pr(Pc

*

)

2

:octadecanol(OA) have been fabricated using Langmuir-Blodgett (LB) technique.A new gas-sensing device——LB-OS chemFET has been designed on which the Pr(Pc

*

)

2

/OA mixed LB film is deposited replacing the gate metal of conventional MOSFET.The sensitive property of Pr(Pc

*

)

2

/OA mixed LB film to nitrogen dioxide (NO

2

) gas is studied by the change of drain current(

I

DS

) during gas exposure.The chemFET consisting of Pr(Pc

*

)

2

/OA mixed LB film can detect NO

2

gas down to 5ppm.The qualities are attributed to the amplification by the FET and the ordered nature of LB film.The mechanism of sensitivity of Pr(Pc

*

)

2

/OA mixed LB film to NO

2

is also studied in the paper.