Heavy crude oil is known to have low hydrogen-to-carbon ratios compared to light oil. This is due to the significant content of carbon-rich species such as resins and asphaltenes; hence their upgrading is commonly through carbon-rejection. However, carbon-rejection promotes rapid fouling of catalyst and pore plugging, yield low upgraded oil and consequently low fuel distillate fractions when distilled. The roles of hydrogen-addition on in situ catalytic upgrading was investigated at pre-established conditions (425 ºC, LHSV 11.8 h-1 and 20-40 bars) using a simulated fixed-bed reactor that mimic the annular sheath of catalyst (CAPRI) surrounding the horizontal producer well of the Toe-to-Heel Air Injection (THAI) process. It was found that with H-addition, the upgraded oil American Petroleum Institute (API) gravity increased to about 5º compared to 3º obtained with N2 above 13º (THAI feed oil). The fuel distillate fractions increased to 62 % (N2, 20 bar), 65% (H2, 20bar) and 71.8% (H2, 30bar) relative to 40.6% (THAI feed oil); while the coke content of the catalyst after experiments are 35.3 wt% (N2) and 27.2 wt% (H2). It was also found that catalyst pore plugging and deactivation due to coke was significantly lower under hydrogen than with nitrogen; hence the catalyst is less susceptible to coke fouling when the upgrading reaction was carried out under hydrogen. The coke fouling further decreases with increasing hydrogen pressure while the API gravity of the upgraded oil marginally increase by 0.3º for every 10 bar increase in pressure from 20 to 40 bar.