LX Ser is a deeply eclipsing cataclysmic variable with an orbital period of 0. ^ { d } 1584325 .
Sixty two new eclipse times were determined by our observations and the AAVSO International Data base .
Combining all available eclipse times , we analyzed the O - C behavior of LX Ser .
We found that the O - C diagram of LX Ser shows a sinusoidal oscillation with a period of 22.8 yr and an amplitude of 0.00035 days .
Two mechanisms ( i.e. , the Applegate mechanism and the light travel time effect ) are applied to explain the cyclic modulation .
We found that the Applegate mechanism is difficult to explain the cyclic oscillation in the orbital period .
Therefore , the cyclic period change is most likely to be caused by the light travel time effect due to the presence of a third body .
The mass of the tertiary component was determined to be M _ { 3 } \sim 7.5 M _ { Jup } .
We supposed that the tertiary companion is plausible a giant planet .
The stability of the giant planet was checked , and we found that the multiple system is stable .