A new charring agent, a derivative of cyanuric chloride, mono-substituted, dimethyl (4,6-dichloro-1,3,5-triazin-2-yloxy)methylphosphonate (CN), was synthesized in good yield and characterized. Its flame retardant and thermogravimetric properties were compared to those of the di-substituted compound, tetramethyl (6-chloro-1,3,5-triazine-2,4-diyl)bis(oxy)bis(methylene)diphosphonate (CN-1), which was prepared in previous work. All untreated fabric showed limiting oxygen index (LOI) values of about 18 vol% oxygen in nitrogen. Fabrics treated with CN at 5-21 wt% add-ons had high LOI values of 30-40 vol%, while fabrics treated with CN-1 at 5-19 wt% add-ons had low to high LOI value of 20-36 vol%. In $45^{circ}$ angle flammability tests, all treated fabrics with CN and CN-1 were passed and some fabrics were not igniting at all. Thermal degradation revealed that onset of degradation and the char yield of CN compound is higher than that of CN-1. Treated fabric with CN, 21 wt% add-on, had an onset of degradation of $240^{circ}C$, while fabric treated with CN-1, 19 wt% add-on displayed an onset of degradation of $230^{circ}C$. Despite the differences in onset temperature, the two samples provided almost the same char yield at $600^{circ}C$, 35 and 36 %. With Fourier transform infrared (FTIR), samples of treated/unburned and treated/burned of CN and CN-1 showed the same functional groups and revealed the disappearance of triazine group and P-O-methyl after burning. Additionally, scanning electron microscopy (SEM) showed that both CN and CN-1 acted as flame retardants by the same mechanism and characterized the surface morphology of the flame retardant treated twill fabrics.