Underbalanced drilling (UBD) holds several important advantages over conventional drilling technology which include minimization of formation damage, faster penetration rate and evaluation of reservoir productivity during drilling. Among many aspects of the underbalanced drilling technology which remain poorly understood, is cuttings transport. An Analytical model to predict wellbore hydraulics in underbalanced drilling was developed and validated using experimental data. Model predictions agreed with experimental data with an average absolute deviation (AAD %) of 10.6.The study of wellbore hydraulics at down hole temperature between 72°C and 270°C using drilling mud revealed that increasing downhole temperature by about 200% caused decrease of 4psi and 48psi in annular pressure drop at 0.5gal/min and 9.8gal/min. respectively. Increasing wellbore diameter by 90% at the same flow rate caused decrease of 5psi and 26psi in annular pressure drop. Also, an increase of 400% in well depth caused 5,890psi and 5,780psi increased in annular pressure drop.